Canvas size question

I have a strange problem when i send my photo i created with photoshop to a company to make a print.
In photoshop i create a canvas with the size of 20x30 (300ppi). I put some photo's on it so it fit almost the hole canvas. Than i have a little space for a nice border around it. Finished. Send it to a company and when i receive the photo i check the size of it. It is exactly 20x30, so that is not the problem.
The problem is that i now miss some peace of my border. So if i wanna keep the border i have to send a photo with a canvas size around 19x29.

What do i wrong ?

maybe some bags ?