A B757-200 Captain received multiple fuel boarded sheets he found to be out of tolerance.

Narrative

We were enroute when we discovered that the fuel sheet was incorrect and the aircraft was fueled with an out of tolerance situation. We computed that we had cleared fuel onboard at point of release. Crew arrived at airport and signed Release #2 with a cleared fuel of 31.0 plus 500 taxi. Fueler came to aircraft and presented the fuel sheet with fuel at 31.5; but fuel sheet was out of tolerance. I advised fueler that this was unacceptable and had to be fixed.Fueler returned to aircraft with Issue #2 of the fuel sheet. Cleared fuel now showed 31.3 plus 500 taxi. We had 31.6 on board. Fuel sheet appeared to be correct; but later we found that it was out of tolerance. We discovered this enroute. With the new cleared fuel of 31.3 vs. 31.0 we requested a new release.Dispatch sent the following message to ACARS 'how about I just update the [flight following reccord]; unless you really want a new flight plan for 300 LBS?' I requested a new release and was sent Release 3.Upon arrival I called the Dispatcher and asked what had happened. He indicated that he had not issued a revised cleared fuel of 31.3 and did not know how it got on the revised fuel sheet. The questions that I would like answered are as follows: If cleared fuel is changed by Dispatch; either up or down; is Dispatch required to notify the Captain? If Dispatch did not change the cleared fuel from 31.0 to 31.3; how did the Issue #2 fuel sheet get changed to 31.3 and who did it?