Book Geometry Logic

Ok, somebody explain something to me. I'm really bad at math and this is really confusing to me, and I can't explain why it happens. Knowing my luck, it's something really easy and right in front of my nose, so if it is one of those things, be gentle with me. I'm a delicate little flower ;u;

.... Books sitting vertically on a cart all manage to fit perfectly.

... but when I lay them horizontally like this :

I can't fit the same number of books horizontally as I can vertically. Now, I don't see any huge differences in there widths. The books' widths are the same both ways. So how come I can't fit as many on the shelf??? In my head I'm trying to account for something to do with the spines, but... that doesn't make sense to me either...

#FIRSTWORLDLIBRARYPROBLEMS

The DVDs don't confuse me as much because we have two different types of cases and they are confusing as hell to put together in a way that is practical or aesthetically pleasing anyway sooooo...... hm.