I thought it was -stein too, but I posit that we all think this way because surnames ending in -stein are very common, while surnames ending in -stain are rare.

I thought it was -stein too, but I posit that we all think this way because surnames ending in -stein are very common, while surnames ending in -stain are rare. We mentally corrected the obscure variant spelling we only see in one place for one we see everywhere else. Hell, my own cousins had a -stein name, so it's definitely something I would've been exposed to a lot. And I always did pronounce it Bernstein as a kid, so my memories are just crap in general.