Thats how the stock bolt should be, but I wonder if the threads are a bit grunged up and the bolt needs to pull in a bit. Its possible it has had the stock replaced I suppose as well.
It is a SILE one piece stock, a lot had them on . You could possibly cut out the stock at the back so it sits lower.
The accuracy on any airsporter is a bit hit and miss depending how good the tap to bore alignment is.
I love them to bits and started off in the 70's with my Dads Mk1 as a teenager shooting at anything that would feed my ferrets, but if I compete with a springer in HFT I use my Pro sport, which is the gun BSA Airsporter should have evolved into.
if I want to shoot something really accurate ie hunting or open HFT, I shoot a PCP.