First, you must have a group. Second, you must have a round that is out of the group ... that is your suspect. Third, when you called that shot, it should have gone into the group. Flyers can be first shot, last shot, or anything in between.
If you know you pulled it, then it is an explained flyer. You smack yourself up side the head for being a moron then go to work to eliminate it. For both kinds, I think back on and then look again -- breathing, natural point of aim, trigger control, recoil control, flags and mirage, was the gun canted, was my eye squarely behind the scope, did I shoulder the gun, did I load the bipod right, are the bags okay, is this piece of brass not the best, round in the chamber too long ... all of that sort of thing.
Only you know if that shot was out of the group when you shot it. Don't lie to yourself. If don't know what that weapon is capable of so I can't tell you if that was ammo or you or whatever but since you said that was the last shot, I'm going with, "you pulled it".
4-and-1 groups are quite common in benchrest. The shooter gets 4 in a bughole and then he gets excited by this fantastic little group and screws up his fundamentals and ... flyer. I have shot a few dozen, maybe hundreds, of those.