Restore Points Disappearing: Allocated Space Still Available. Why?

I have allocated approx 18gb of space for system restore and it is currently sitting around 12gb.
My question is, why do I only have 2 days worth of system restore available? When there is still more space, I would expect it to go back a bit further and the way that I understand it, Windows 7 keeps storing them until it is out of allocated space.
Anyone know what is going on?
Thanks!

huza said:

I have allocated approx 18gb of space for system restore and it is currently sitting around 12gb.
My question is, why do I only have 2 days worth of system restore available? When there is still more space, I would expect it to go back a bit further and the way that I understand it, Windows 7 keeps storing them until it is out of allocated space.
Anyone know what is going on?
Thanks!

If they are 6 gigs each and you have two=12 gigs your space 18. One more wont fit.

I have 12 restore points at the moment, going back 3 days. That would mean each point was much smaller than 8gb. While it is possible that the next one is massive, I have made no big changes so that is unlikely.
Thanks!