It doesn't matter, the 11-1/4" will work. The hens are not going to borrow a tape measure from you to check out the size. Growing up on the farm I saw hens make a nest in some areas so small many people on this forum would not believe it. I once used a cat litter bucket as a nest, the top was 7-1/2" x 11-1/2". Hens the size of Australorp, Delaware, or Sussex laid in it.
Many of the numbers you see on here are not what is "best" or minimum requirements, they are general guidelines. People starting out don't have the experience to know what size to make things or what numbers to use. I'm talking about nest size, coop space, run spaced, roost length, number of eggs to put under a broody hen, practically anything that involves numbers. You can see some really strange numbers on this forum, but most of the numbers that are generally agreed upon are large enough to keep most people out of trouble most of the time in almost any circumstances without being extravagant overkill and overly expensive.
The 12x12 nest box is that way. Practically any chicken can lay in it, even a huge Jersey Giant, especially if the top is open. It is obviously bigger than it has to be for a bantam or even a smaller full-sized fowl like a Leghorn.
So do not worry about your nest, they will work fine.