Think of it like this, 5 people (5 miners in the pool - slush) are standing in a room and are being asked to guess a number between 1 and 10000.
They shout their guesses out as fast as they can.
They are trying to guess the same number faster than the 50 people in the room next door (50 miners in ghashio).

With 50 people compared to 5, it's most likely that the larger pool will guess more numbers.

On a lucky day however, slush with it's dedicated army of 5 - will punch way above it's station (and historically always has).

On unlucky days the 50 people next door are having all the fun and us paltry 5 people seem to have no chance.

TL;DR version for the impatient
So to answer you question, the reason we have these longs rounds is due partly to bad luck and partly to the fact that there are much bigger pools than us. I still mine here, always have liked the underdog.