@Nicegot I think it's one of those things that's difficult to translate. Sempai says she doesn't like tomatoes, but with a kansai dialect turn of phrase that people from other parts of Japan wouldn't necessarily understand; Kouhai picks up on it, though, which to the Sempai is both surprising (that he'd start picking up bits of her dialect) and a bit embarrassing (she absentmindedly slips into her home dialect whenever she's comfortable enough with someone or a situation not to put up a professional front). He's happy that he understood her and doesn't want her to feel embarrassed about her speech while she's a little flustered that she let the mask slip some in front of someone with whom she's personally comfortable.