Two measurements cause most framing problems, and neither of them is the size written on the order.

The rebate is a depth, not a width

It is the lip inside the back of the moulding.

Everything going in has to be shallower than it: glazing, mount, artwork, backing.

Which is why canvases and objects go wrong

A stretched canvas or a mounted textile has real thickness.

Ask for the rebate in millimetres against your measured stack.

Sight size is smaller than artwork size

A mount overlaps the sheet by about five millimetres on every edge.

So an eight-by-ten print in an eight-by-ten aperture loses its border.

Write down all three

Artwork size, sight size, outside size.

One of those numbers on a reorder instead of three is how the second batch arrives different from the first.