Unusual problem with compound mitre saw

I have an annoying, unusual and potentially dangerous problem with my Makita MLS100/2 250mm Compound Mitre saw

Ive never noticed this before and its only had light usage over the space of a couple of years, but i cant understand how a) i could never have noticed it before or b) how it could have happened since i got it.

the guide at the back which you hold the wood against seems to me to be totally off... when i first noticed it (as the wood was jumping badly when cutting it) i assumed the wood was badly warped, but i checked this and it isnt the wood., i have attached a photo which is hard to make out, but when pressing the wood against the back rail, then there will be an open gap int he centre asif the 2 parts of the rail are squint, so when the wood is cut through, the blade forced the wood to jump backward to hit the rail.

its not apparent or a problem when trimming a small bit off the end of a bit as the wood will sit flush when pressed again one side only...

but of course the problem here is that i have no idea if it is cutting even remotley accurate angles...

in the 2nd attached image i have tried to draw a diagram showing what i mean, in A a long peice is held against the rail creating a gap, in B a shorter peice is held and is ok, but potentially not at a 90 degree angle...

Please forgive me if i am being stupid here but what is going on? as i said it has had light use so i cant understand what would cause this.

 

I can see bolts on the fence, but the fence sides are connected by a cemi-circle at the back so don't know how I could adjust each side to staighten it...