Photoshop is FAAAAAAAAAAAAAAAAAAAAAAR from pixel perfect or accurate tool.
I mean, Photoshop doesn't even know how to mirror or turn stuff in 90 degree angles and the problems have been documented. Adobe does not know how to code simple stuff to save their lives/company and it shows.
So, you just probably stumbled on some rounding error of some sort. As far as I know PS uses inches as its internal units so rounding errors are plentiful.