Hammer measures

Check if you have snap to grid unchecked.

Are these brushes that have at some point been moved while a prop was selected at the same time? Props are often offgrid and if you move or rotate a group of objects that includes props the bounding box will often be off grid and that will send the props on grid and the brushes off grid if you drag it around.

To avoid situations like this (and because it's so damn useful) I use the arrow keys to move objects. There is a checkbox in tools->options->2d views "Arrow Keys Nudge..." to enable this. It also works in the 3D view.

it is prolly just props throwing it off,

if you need to move a bunch of brushes and props jsut select them all and turn props off in the vis group they will move with everything else but it wont try to snap a props off grid bounding box to grid that way

when moving a brush, it uses the corner closest to the mouse for grid snapping. So if you have a brush which doesn't fit perfectly into the grid, depending on which corner snaps to grid, it can get misaligned.