How accurate is A99's Face Detection?

In the last weekend I took some outdoor portraiture for my kid, with A99 and 70-200G. I noticed one thing: when Face Detection is on (by default) it sometimes override my AF selection, cause blurry image (out of focus). When the big Face Detect green square was consistent to my AF point I still get some blurry shots (not always but more the a few). I don't know for sure what caused this, as soon as I turned Face Detect off almost all the pictures came up sharp again. I used AF-S at first, but later on switched to AF-C since my boy was constantly moving around. I don't think AF-S or AF-C made any difference there.

To A99's credit, these in-focused picture were very exposed with perfect WB (something my old A900 is not always good at). My 70-200G seems to be little sharper on A99 than on A900 (both micro adjusted).