Section and header/footer border inconsistently scaling

Site spacing style is set to 1400px max width, 3vw site margin. Header is inset.

I have two sections on this page (site is herring-salmon-ezme.squarespace.com/560. pw: testtest.), one with a gallery block, the other with text blocks. All blocks are dragged as far right as possible (right alignment).

On fullscreen, section right hand border is aligned properly with the header right hand border - all good. But when I minimize/scale down the window, the section right hand borders are no longer aligned properly. How can I keep section border aligned with header border as window size scales down?

Images attached. any help greatly appreciated!

thank you! i did not realize that snippet was affecting content site-wide. i needed to limit it to the intended page.

2 minutes ago, stickarch said:

i needed to limit it to the intended page.

Yes, do you need help with that?

I'm all set, thank you.

I do need help with a separate issue on the same page, if you're offering.