All cut film sizes are nominal, they are based on the earlier glass plate sizes but are a fraction smaller, this was because originally they were used in plate holders with a thin metal adapter. There is an ISO standard for all film sizes.

So Ilford 10x8 photographic paper wont fit in a 10x8 film holder unless you trim a couple of millimetres off the edges.

A few years ago J&C in the US had Chinese film packaged for them, they failed to take the size difference into account and had many unhappy customers whose 5x4 film wouldn't fit there dark-slides.

Ian