"Another stat which ties in is that compared to fibresand which was at Wolverhampton and still is at Southwell, the mean distance between first and last placed horses has nearly halfed on polytrack, ie horses are finishing much closer to each other and there are consequently tighter finishes. "

This could be race development issue like we see on turf.

Given that front runners are doing so terribly, the jocks would tend to get less aggressive early to improve their chances of winning. When everything is slowed down like that, the last part of the race is quicker and all the horses are relatively fresh. The margins would tend to be smaller because the better horses can only outrun the weaker ones by so much within that shorter period of time they are working harder. Also, with a less aggressive race, the weaker horses wouldn't be used up chasing the better horses early.