I think your figures are just a little on the low side there! From the last quarterly results:
- 18.6 million iphones sold.
- 9 million ipods (remember they share the same screen as the iphone normally), 50% were touches, so call it 4.5m
So, 23.1m devices in 3 months, or 7.7m per month. Even without the ipod, it's over 6m/month. And remember this is 6 months after the iphone 4 launched, and not a high sales (as in pre-christmas) period. Add a new device or holiday sales boost and you can expect a lot more than that.
Also, from the results: "189 million cumulative iOS device sales by end of March quarter". 15 million were ipads, so 174million iphones + ipod touches.