Janet and Hope's surnames in the MCU are van Dyne yet Hank's is Pym. Janet is married to Hank and Hope is their daughter. It is usually the case that a wife will take their husband's surname upon marriage and the daughter will then take that name as well.
I've seen the theory that Janet didn't take Hank's surname when they married which is entirely possible but there's no direct/explicit mention of this that I can find.
The theory on Hope is also that either she has always been a van Dyne after her mother or she changed it from Pym after her mother's "death" and her father becoming distant in a way to honour her mother and distance herself from her father.
Hope van Dyne: You know when my mother died I didn’t see him for two weeks?
Scott Lang: He was in grief.
Hope van Dyne: Yeah, so was I, and I was seven. And he never came back, not in any way that counted. He just sent me off to boarding school.
However, there is no direct/explicit evidence I can find which back either these up except informed speculation.
Why do they have different names?