Last week I designed 40th wedding anniversary invitations for a lovely woman from New York. She was amazing to work with and we finished them in less than a day. This week she contacted me because the online printer she was using was giving her an error message. It didn't take me long to see the issue. The files I designed were for 5 x 7 flat cards.
A 5 x 7 card from this printer is actually 7.17 x 4.61. I guess it is just easier to label them as 5 x 7 and think that it is "close enough" but to a designer it is completely wrong.