A few years back, Pro Football Focus got in on the list-a-palooza tend that was sweeping the NFL, and they debuted their Top 101 Players. I’m guessing they threw in that extra 1, so they could stand out from the other Top 100 lists floating around.
Another thing that PFF tries to do to separate themselves is they take an “all positions are created equal” stance when compiling their list. The theory is they won’t have a list full of quarterbacks and “skill” players up near the top like some of the other lists out there do.