737 Autobrake PI distances
I believe the autobrake system applies the brakes appropriate to achieve a set rate of deceleration and all but Autobrake 1 applies continuous braking.
Applying reverse thrust decelerates the aircraft also (I am aware at different rates at different speeds), allowing the autobrakes to brake a little less to maintain the same rate of deceleration for the set autobrake setting.
If on landing you forget to select idle reverse, I would therefore assume that the brakes would have to work a little harder to maintain the deceleration rate.
The Performance Inflight landing distances (except Max Auto) have an addition to be made for not selecting idle reverse, increasing upto over 1000ft for Autobrake 1. (-800)
Thus far no one I have asked knows why, and our manuals on the autobrake system give no answers. So any insight would be appretiated
Many thanks in advance