The pasuk says that Noach was tamim “b’dorosav”. Rashi says: Some of “raboseinu” infer praise (of Noach) and some infer criticism. Those who infer praise say that if he was so great in *his* generation, when he was alone, imagine how great he would have been if he had been around people like Avraham! Others infer criticism – only in his generation could he have been considered great, because there was no one truly great. But if he had been in a generation with people like Avraham, he would not have stood out at all.
The question is, why are these p’shatim mutually exclusive? Likely both are true – “ba’asher hu sham” he was not on the level of Avraham’s generation, but if he had been in Avraham’s generation he would have been far greater.
Looking more closely at Rashi, perhaps this is what he means to say: there are two aspects of this word that one could choose to focus on. Those who focus on the positive aspect are Raboseinu. Others, people of little significance, focus on the negative aspect. This is what Rashi means when he uses the word raboseinu only for the first inference.