I was looking at the Billboard Top 40, which tracks airplay, whereas you are looking at the Billboard Hot 100.
An interesting fact about the Hot 100 was that. at that time a song could not be on the list until it was released as a single in the US, and "I'll Be There For You", for whatever reason, was released as a single AFTER it had peaked on the Top 40. At that point, I'm guessing people had either purchased the Rembrandts' album or the Friends soundtrack, which probably ate into the sales of the single.
And so you have a situation where a group's best known single was not their highest charting single on the Billboard Hot 100, yet is inarguably their biggest hit by virtually all other metrics.