Media query acts inconsistently on exact width

When I set the query max-width to 639px, when screen size is exactly 639px, the styling isn’t used.
When I set the query max-width to 640px, when screen size is exactly 640px, the styling is used. Since I’m using srcset with some images on the site instead of media query, my website is broken on that exact width no matter what. Why?
This is the website: http://gostartupsdev.wpengine.com/

Only developers drag the edge of their browsers to watch media queries work. That’s not a realistic environment. It works fine in Chrome devtools responsive design environment using presets or dragging the window around. I checked your CSS source for your media queries BTW Sometimes after a long session your browser gets tired and you have to cross the media query limit. It’s good here.

Thanks. I won’t fret over it