Continuity mistake: Jamie and Paul are at the supermarket, and Jamie is eating chips. She leans on a coin operated horse. The bag of chips in her right hand changes from front (Green Label, "Baked Lays") to back view in one shot then to front view again.
Deliberate mistake: The first 10 episodes of Season 5 were shot in a totally different order than they were aired. As a result, Jamie's size during her pregnancy is very inconsistent at the beginning of the season. In particular, she gets much smaller between episode 8 and episode 9 because episode 9 was originally supposed to air earlier in the season.
Answer: Just think about your own life, I am sure you do not act the same with your siblings or close friends as you do with a stranger (because that is what the Buchmans essentially are to Ursula). In Mad About You, she is an extremely bad waitress and in Friends she is a jerk - those are not mutually exclusive character traits. This can also be seen in the Friends episode when Jamie and Fran are in Central Perk and run into Phoebe who answers their questions in an aloof manner and they assume it's Ursula. Is Phoebe always aloof? No. But she was to strangers asking her weird questions. Also, Ursula has been aloof with Phoebe as well (like in the one where they all turn 30 or the one where the grandmother passes away, etc).