Why are there sometimes 15 new posts and sometimes there are 10 pages of new posts?

Sorry, I'm not understanding what you mean by that.

When you click on "new posts" sometimes there are only 15 posts. Other times there are 10 pages of posts.

Depending on when you were online last. Sometimes if I'm offline until the next day there are close to 10 pages of new posts.. but if I come back in like 2 hours, there's only 15.. it updates everytime you come online.

It will also depend on if you are actually logged in or not.....if you are not logged in then everything from that day will come up, but if you are logged in and you were just logged in a few hours before then you'll just get what's new from the last time you logged in....does that make sense?

Thank you, all!!!

Today when I first logged in there were only 9 new posts so I'm still not sure I get it.

Try checking Today's Posts before logging in.............I generally don't log in unless I am going to respond to a thread or start a new one. I can get alot of reading done before logging in and the search goes back further too.
Hope this helps!