It could also apply to the wear on the chainring and cog if their teeth are in a perfect proportion to one another, but without thinking too hard about it, I'm

It could also apply to the wear on the chainring and cog if their teeth are in a perfect proportion to one another, but without thinking too hard about it, I'm guessing that's actually rare. For instance a 2:1 ratio would be awkward for most cyclists riding on pavement (too low) and 3:1 too high, if they had to choose exactly one ratio. I ride 46:19, because those are the parts that were in my bin when I built the bike, and it's a pretty good all-round ratio for city riding unless you're a lot more athletic than I am. Also, those components wear out soon enough anyway -- a chain lasts 2 to 3 thousand miles, and a cog lasts a few chains.