Rashi, on 173a in explanation of "meshulashim" uses Yosef ben...Yaakov instead of the usual Yosef ben Shimon. Twice. Why the change?
The generations in the "Ben Yakov" example are well known. Using that system we can go up to four familiar generations, conveniently using "Avraham."
This begs the question. The Mishna could have also used that example, and chose not to (I remember seeing somewhere a reason why those two specific names Yosef ben Shimon were used, but can't remember). Why did the Rashbam decide now to go against whatever reason the Mishna had?
The Mishnah does not *name* more than the first two generations (i.e. Yosef and Shimon). The Rashbam, however, is interested in listing the names of the third generation and up, so ben Yakov is more convenient.