It's not quite as you say.

There is a thing called variance.  That's why some blocks are "lucky" and "fast".  And some aren't quite so lucky and fast.  In fact, some are really really bad.  Just like some are really really good.  If you can have a 2% luck block (#2176), you can certainly have really long blocks.

It's well known that the more hash rate a pool has, the less variance comes into play.  Conversely, the less hash rate a pool has, the more variance comes into play.

M