Mmm...If I had to make an unqualified guess, I'd say that you were causing the error by tension in the extraocular muscles, and the yoga relaxed them.
See Wiki
What astonishes me, is the error can be very constant...almost as if some feedback signal is giving wrong data and setting the eye to the wrong focus. Only when the entire system is truly at rest, can an optician make an accurate assessment of your prescription. Happily, it seems that not many people are thus affected.