Depends on the work item Chrissa - some changes cannot be done in full at the time and may need to be carried over - the systems that all government depts use are old and that is why they keep sending out stuff.
For your own piece of mind just give them a buzz - FGT
like you Chrissa, I once had 3 differing letters, on phoning them, my benefit entitlement was marginally greater than any of the letters.
Only reason I can think of is because I was able to give them accurate info which wasn't covered on the C of Circs form.
Two of them may relate to different billing years. I recall the same thing happened to me. We also now get sent two copies of everything- one for me and one for mrsfactor- even though I pay the DD