It seems that when you log into your online profile again after you've reinstalled Windows, that the achievements that have a simple "achieved" state are successfully remembered, but achievements that have a counter that hasn't been filled (rather than just achieved or not flag), like Cryomancer and Pyromancer, get reset to 0. The last time I logged in I was using 1.5.5.
I take it that this is because the counter is stored in the "C:\Users\<Username>" but not actually in the online profile, unless you've achieved the requirements?
I'll ask this on the Steam forums too so the information is there too.
