[maemre]

In a lot of math disciplines, the papers follow the Hardy-Littlewood rule, so the author names are ordered alphabetically [1]. Maybe, that's what you've been noticing. In my area (programming languages, which may be sometimes theoretical but it's mostly a mixed bag), I noticed only one group follow that convention. Others follow the "first author is the main contributor, last author is the advisor" convention you described.

[1]: https://en.wikipedia.org/wiki/Academic_authorship#Authorship...

[shrimpx]

I double-checked and it looks like you're right, that the Theory papers where I thought the proof took first-author are actually alphabetical.