Why do i hear a difference in siting and singnig and standing and singnig. also will I feel different engagement of my lower support mustles

It's easier to stand with great posture than it is to sit with great posture for most folks. Your lower body is engaged to stand and can be fairly inactive while seated. Singers in choir rehearsals typically sit for the majority of practice, but they scoot to the front of their chair so their feet are on the ground and their lower body is more engaged. You can try that to see if it's easier for you!