THERE ARE TWO specific seat fitments: ONE seat in multiple configurations that fits all the "standard frame" XB models which are the S...SX...L...SCG. ONE seat in multiple configurations that fits all the "long frame" XB models which are the Ss...Stt....X...XT...XP. STANDARD and LONG do NOT interchange.
here is a properly fitting seat on a 1000 mile-from-new late model SX. your S should fit precisely the same.
cherry2 (4).jpg
cherry2.jpg
three things come to mind that could affect correct seat fitment:
1-incorrect model seat for your standard frame XB
2-front steel engagement tab that locates and secures front of seat has been substantially bent.
3-rear seat locking latch has been bent or improperly adjusted
4-seat pan being interfered with by something askew such as the ECM...incorrect battery...loose battery....an idiotic and ill-conceived under-seat modification.
5-a bent/loose/tweaked/modified rear sub-frame assembly.
CHECK THEM ALL