All my life, oven temperatures have been given in multiples of 25, and oven dials were labeled that way. Now I’m seeing “Preheat oven to 315 degrees” or " Preheat oven to 365 degrees". No oven is that accurate/precise, so why such detail?
Could the recipes have been converted for the American market from original Celsius temps? Certainly when I convert the other way, adjusting to say the nearest 5 degrees I get some equally unusual figures.
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I think John may be on the right track. There might also be a factor of more digital controls for ovens with more precision. Note that precision and accuracy are very different things. Consumer ovens are notorious for being WAY off the setting you choose.
I know the difference between precision and accuracy; my oven is neither. And I should have noted that this is on packaged food, not recipes per se. Somehow the numbers don’t seem as if they were converted from “round” Centigrade numbers, but I suppose it is possible.
I wonder if it’s like the cake mixes that used to say, “Add one cup plus two tablespoons of water” because that make the cook feel as if they were really baking the cake themself.
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I’m not sure it’s a conversion issue.
Consider that 315 F converts to 157 C.
Are there really recipes that call for baking something at exactly 157 C?
I think this is the reason.
Most, if not all, modern ovens have digital temps, which just sort of feeds the fetish of cookbook authors of being oh so precious and precise with their instructions.
In fact, I can’t think any of new ovens that do not have digital temperatures readouts.
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I’m not surprised in the slightest Evelyn. My oven is quite inaccurate but disturbingly precise (1F). I collected a bunch of data from an oven thermometer (which I checked against an ice bath and boiling water as part of my attempt to interest nieces in science. A little curve-fitting and I developed quadratic equation for settings. My wife was not amused (digression: there is no science in New Jersey - I’m sure I’ve posted on that topic before). I built a little conversion table which has boiled down to “set 15F high below 300F and 20F high above 300F.” Close enough.
I try to think the best of people and the temperature scale conversion is silly but doesn’t point fingers at anyone directly. If you’re correct, and I fear you are, undue precision is probably a factor of people trying to show how smart they are. In the end they achieve the opposite.
Only those which otherwise require baking at exactly 315F.
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Good point.
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Sous vide recipes often call for precise temperatures. People discuss the differences between a 64C and 66C egg, fish at 116F vs 120F, steaks at 125F or 130F.
(I haven’t tried eggs yet, but for me it’s fish at 116F, steaks at 128F, chicken breasts at 147F)
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This isn’t some fancy sous vide recipe–this is instructions on how to heat up a cheap frozen Aldi stromboli. 
I only ask because this is not the first time I have seen this. (Come to think of it, the most recent other occurrence was an Aldi frozen pizza, so it may be an Aldi thing. The items were produced and packaged in the US, so temperature conversion seems unlikely, but possible, I suppose.)
My oven is old enough that it had the old-fashioned analog temperature dial.
And then there are gas numbers. Cooked with those a few times, but leave the discussion to our EU/UK friends.
Can you recruit your nieces to figure out a formula for translating microwave times, always stated for 1100 watts, to my tiny 750 watt model? 
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I’m thinking since microwaves make molecules vibrate and molecules are directly proportional to volume - no wait - that doesn’t work. Hmm. How does 5/3 work with your experience? If that doesn’t work let me know and I’ll dig into it.
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Right, but no one uses an oven to sous-vide.
I’m going to need footnotes from a credible academic/scientific source that 1F makes a difference in sous vide. There are too many independent variables: insulation of the container, ambient temperature, solar loading, viscosity, salinity, … I need real science to believe it.
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I’m only reporting what I have read. I’m also skeptical about a 1F difference, but the difference between 116 and 120 for fish, and 125 and 130 for steak, is noticeable.
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If you cook at the higher temp for less time, what is the difference?
Isn’t that defeating the method of cookery?
My point is that if 1F or 5F makes that much of a difference, is there a time factor? 115F or 116F? Really? 115F or 120F? Again - footnotes.
