Alliance tower, something wrong?

I’ve reached alliance tower liv 6 and my daily donation increased from 20k to 50k, so it’s a +30k.

 

Now for the next level the donation will arrive at 75k. So the increase is of +25k, less than the last upgrade, and liv 7 costs 400 gems.

 

Can anyone tell me why?

Yes, It’s right… not all differences between the previous upgrade and the next upgrade must be more high.

To know all costs, time and upgrades of Alliance Tower check this page : http://royal-revolt-2.wikia.com/wiki/Alliance_Tower