I got this answer from NEUCOIN:

When staking amounts like 10000 NEU you'll have much less than one percent chance of minting coins each day. So you could go months without minting coins, and you're not going to get such a compounding advantage as your calculations. You'll also have a lot of uncertainty, if you're unlucky it could be years before you mint coins. The good thing about the growth accounts is you're guaranteed to get coins.

That said I don't like the way the growth accounts work. It would be much better without different time periods. So you could transfer in and out coins when you want, and get paid rewards on a regular bases. Maybe someone else will implement a more user friendly growth account this way.