Don't some autothrottles have a tendency to 'overreact' sometimes to events like loss/gain of airspeed?...perhaps in the gusty conditions at the time the autothrottles had just retarded almost to idle after a large gain in airspeed and then failed to power up again? Presuming they were flying a normal stable approach until 600ft the power setting they would presumably have had (even if the engines did get 'stuck' at that setting) would have been enough to carry the plane a bit further than it actually went. Possible combo of windshear and unresponsive engines?