WordPress can't automatically fill space with a varying number of posts -- it uses the "Blog pages show at most" setting under Settings -> Reading to determine how many posts to display. By making posts Sticky, they're taken out of order and added to the "Blog pages show at most" number.
Suburbia has a unique grid layout for displaying posts and Sticky posts -- the first row displays 4 regular posts, and following rows display 5, and Sticky posts take up 2 regular post slots. This causes gaps on the home page or subsequent pages when Sticky posts are set because the math never works out.
As I see it, you can:
* Not set any Sticky posts, and set your "Blog pages show at most" number to 9 -- this fixes the uneven posts problem
* Set however many Sticky posts you want, and ensure your "Blog pages show at most" number rounds out the gaps on the home page -- subsequent pages will look uneven
* Switch to a theme with a different post layout -- there are grid-style themes that display posts + Sticky posts evenly if you set the correct number of Sticky posts
I hope this helps!