Bingowings said:

It's never been properly established where in relation to the base the generator is.

Presumably it's some distance away from the base or the Rebels would have detected the probe droid much earlier but also presumably it's near enough to efficiently be serviced by the Rebels.

I think your right that Veers starts off with the pack but then heads for the shield alone... Which would make the count 6.

3 for the ground assault front of Echo Base and 3 for the shield generator... So 2 AT-AT's get taken down on their way to the shield while the ground invasion AT-AT's take no casualties from the trenches and Rebel gun placements

Obviously there is a 3rd front with no AT-AT's at all where Vader lands which we see nothing of

The thing is it's so jumbled that it's like you are watching bits of a planned to be filmed ground assault but your not also.