In the case of an inertial feed that presents as a double feed, the pistol recoils to the rear with fast slide velocity, causing the top round in the magazine to Move forward in the magazine as the pistol is moving rearward. Think glasses on a table as the table cloth is pulled away. If the round moves forward enough it can begin to chamber while the slide picks up the second round (if it has the opportunity to rise to be contacted by the feed rib, which it often does not)... this looks like a double feed, and I suppose it sorta is, but it's more accurately described and diagnosed as an inertial feed.
