RAT 5: That's a good question and one that puzzles me too.
A quick example: We were coming back to the UK last night and got a good direct from the Polish border to OTBED. After selecting 'ABM' on the legs page and tidying up the mess, I noticed that the track between the remaining waypoints varied by up to 3 degrees.
FCOM2 explains:
Data and status corresponding to the parent waypoints is passed to the abeam waypoints. If abeam distance is less than 100 nm, only the wind data is passed to the abeam waypoint. I haven't noticed the effect of abeams points on the SAT but will have a look this afternoon. Oh wait, my mistake, we're flying to Spain so we're unlikely to get any shortcuts... 