Moving from Party 5/10 up? -- Coffee Talk

Oh jeeze...I'm feeling a little verklempt...talk among yourselves. I'll give you a topic:

If you look at party 10/20 table lists on a typical night there are a few full ring tables, a few more 9 max tables, and then 6 max (aside: 9max...uh..ok). However, it looks like there are significantly many more full ring 15/30 games. Do most people skip 10/20 on their way up and go straight to 15/30? In other words, what's the deal with 10/20 on party? What about 15/30 --> 20/40 --> 30/60? Do most people skip 20/40 here too? I'm confused...

Discuss.