Well, we knew that the combined 2nd and 5th fleet that took back DS9 weren't full examples of the entire fleet. The combined task force had elements from each fleet, but these fleets had remaining ships that did not participate in the battle. My personal opinion is Starfleet has a system of preassigned, informal fleets either for certain purposes, or simply assigned to them to allocate chain of command. What we saw with the 2nd, 3rd, 5th, 6th, 7th, 9th, and 10th is they were the ones that were essentially beefed up for major combat missions and offensive battles. Perhaps the 1st, 4th, and 8th fleet were essentially cannibalized and had many of it's ships transferred to these fleets to boost their combat abilities, while the remnants of these fleet served minor roles. My guess is the 1st fleet was cannibalized, and the remnants served diplomatic and escort roles during the war. The 4th and 8th could have been specifically assigned as defensive fleets to protect the multiple shipyards.
Additionally, I agree that these fleets might not have even existed outside of the Dominion war. However it's also possible that there were a lot more than 10 fleets in the Federation. What we've seen often in Star Trek is they tend to prefer using Task Force as a way to designate a group of ships more often than fleet.