Lionel Debroux (./9) :
According to http://mobifiles.bytefox.de/viewtopic.php?f=25&t=3632 (in German), the sqrt function is faulty (and has been since at least 0.81, probably ever since the function got first implemented), it computes sqrt(x/10) instead of sqrt(x).
(Source: http://tichessteamhq.yuku.com/topic/4703 )

Note: I highly recommend using zs for all numerical computations. It is much more tested than the functions of the default interpreter.