In the episode Ross is talking to Joey and Phoebe before he plays Rugby and he talks about Red Ross and none of them remember. He said it was when they all were at the movies to see Dances with Wolves and someone cut the line and his face went all red because he was so angry.

Dances with Wolves came out in 1990 and the pilot episode was in 1994 and in that episode Ross and Carol were just divorced and Carol had moved her stuff out of his place. In The One With The Flashback in Season 3 it placed Joey just moving in with Chandler just as Ross found out Carol was lesbian. There wouldn’t be a way for Joey to remember Red Ross since the movie had came out 3 yrs prior to him even meeting the group.

Possible explanations:

1.They simply saw a rerelease in a smaller theater of Dances with Wolves after its theatrical run. So Joey would have been with the group at that point

1. Ross was with Carol for a few years after he found out she was lesbian and didn’t divorce her soon after he found out but this would also conflict with the other established timeline in The One With The Flashback because Rachel was about to get married to Barry in that episode and in the Pilot she appeared in her wedding dress. This could possibly be explained by them pushing back the wedding date.