My best guess might be truck size? Companies that use enormous trucks (like Atlas) will sometimes charge you for extra unused space if they can't fit another load in the space that would be leftover...but I wouldn't expect that much of a difference.

I thought about going with PODS when I moved out here, since I was in a furnished apartment for a few months until I bought my house. But in the end, I just moved the stuff in a uhaul with my mom and then put it in storage until I found a place.