Snap to Grid Not Working.

Hi folks
Latest version of Photoshop & Mac IOS.

Grid on, snap to grid checked. When I create a marquee or shape, dragging and letting the edges 'snap' to the grid, it's always .1mm too big. That is, a shape created using a 25mm grid becomes 25.1mm.

It's very subtle, so easy to miss and has been creeting all sorts of issues.