Why does changing the settings of a mask only change another?

[William24921499e1xx]

I created an unlisted video to explain the issue.

https://tinyurl.com/2jzrwd6s

(made an error in the text of the video, changing size of mask 1 actually does change size of mask 1, not 2, but everything else still applies)

[William24921499e1xx]

Heres the video url in normal format: https://youtu.be/NdbBQNAyPYI

[William24921499e1xx]

posting it again, idk why the youtube has a '.' in the middle of 'youtu' and 'be'

https://www.youtube.com/watch?v=NdbBQNAyPYI