How are mined coins picked?

What determines when the miners jump coins? Like right now Novacoin is killing it according to whattomine.com but seems like I see a lot of game credit and bitconnection being mined.

Coin difficulty is a huge factor.
What to mine is probably showing 24hour mean difficulty. Prohash is using as close to current diff.
Also WTM probably does not even show some of the coins this pool has?