ok. I understand now.
I don't think the resizers are meant to adjust screen resolution.
They are only meant to display a visual representation of how your screen will appear within certain media queries / resolutions.
The four device icons at the top of the screen adjust the media queries (resolution).
Notice if you select the Vertical Tablet icon - then use the resizers... the bottom center of the screen will display a list of devices compatible with the selected resolution. The selected media query however never changes.
Think about this... if they allowed you to change the top border of an element at 900... and 901, and 902... 950... 960... etc...
you could potentially have 1 value per pixel resolution... for the top border of 1 element.
Needless to say... your css file would contain some weight