What determines the camera roll image numbers?

When you add a picture to your camera roll, it's name should be IMG with a number next to it. So for example, if I start with an image it's called Img_0001, and then when I add another picture it's called Img_0002. I know that if I delete Img_0002 and take another picture, that the new picture will just skip ahead and be called IMG_0003.

However, when friends send photos from their camera roll to my Iphone, I notice that their Img count seems to be all over the place. A friend sent me 2 screenshots that were taken 5 minutes apart and one was Img_0342 while the other was Img_5455. That's a MASSIVE jump. And I know that one of my friends just recently got a phone and yet one of the selfies they sent me was Img_6234. I find it hard to believe they've already amassed 6000 pictures.

It's a minor thing really, but I'm just curious as to what causes this? Is there some kind of explanation? Do the numbers hold greater meaning? Is it random? I like to keep my photos organized and I could imagine something like this causing an issue if it ever affects me.