A 30px high sprite won't fit into 30px high gap since it would be overlapping the sprites above and below. If two sprites share an edge they count as overlapping.
I'd readily accept that, but can I ask how that relates to my example, here...
<img src="http://img268.imageshack.us/img268/1112/zlsu.png" border="0" />
My player, blue, moves along the red base freely even though it's sharing an edge along the bottom, but wont go any further into the 30px gap even though at this point it's not sharing an edge with the red platform above-right? (...actually, is it because any further movement right WOULD trigger a collision, so prevents it?)