why is BP different between standing and sitting?

my boyfriend (20m 5’10 large guy) asked why he can’t cross his legs while taking his BP. i said it makes it not accurate i think but i don’t know. we were testing out a at-home-machine my tia owns and his BP while sitting was 105 over 63 but it was 132 over 96 while standing. he takes sertraline and occasionally has an edible but was sober at the time. his bp is like always perfect or like a little below. which he tests pretty often cause he does when my tia does cause they like to compare she has hypertension and he’s normal. ok he just texted me and said he has a lot of hypertension in his family (dad, mom etc) and doesn’t know if that’s relevant to him.