Mission Generation Times

So i am slightly confused, I have a premium Account, I am on turbo, I have 36 fire stations, 11 Pd stations, 8 hospitals and 6 Ems stations. Yet even with only 11 active calls I get a new call every minute or so and rarely get above 20 calls. This dosen't seem right to me? does it take into account shared alliance calls? what is causing me to generate less calls, Do storms or other events on the map affect individual call volumes?

Previously i have read that the limit is # of fire stations+1 and i have had up to 28 calls in the past so I don't think that is it. My real issue you here is that the major benefit of premium is the turbo mode and as far as i can tell i could get the same calls on fast at this rate... seems premium is a bit of a waste if that is true.