I had one of these, after my third season in the Premiership three random players appeared in the overall best 11 that I'd sold on previously.
I think I worked out why though - players who were previously on the list who I'd been playing regularly have had their average rating dropped due to the higher standard of competition. Someone who got 7.5 in the Championship, then moved to a bigger club then goes back on for someone who got 7.6 in the Championship, then 6.7 in the
Prem to bring their average down, meaning an older player holds the better average.
You're playing in Northern/or Ireland, are you not? So would the all-Ireland cup maybe be playing havoc with the appearances then? The FAW Cup for me is making adding up overall appearances awkward.