Lightboxes behave differently

I have 2 custom lightboxes, one for signup and one for sign in. On the signup part, its perfect nothing is wrong. but on the sign in page, i cannot adjust the bottom part of the lightbox (red circle) so if i make it smaller it will leave a gap on the upper part.

i believe this caused on mobile version, the sign in lightbox slides left and right, not like the sign up lightbox which stays in place, it really look unprofessional.

what should i do? thanks in advance!
(sorry for my bad english)