This is a rare issue related to browser limits in elements shaping. In fact Media Grid calculates in a very precise way element sizes, but browsers are only able to render integer values.
The consequence could be a “1 pixel” error on some resolutions.
Normally can be solved tweaking the grid wrapper: adding a DIV element slightly smaller than original container.
<div style="width: 99.5%;">[THE MEDIAGRID SHORCODE]</div>
do that in WP editor, “text” mode.