Obito is right. For balancing purposes, a ki of 250 is required to fly. And the level cap has been reduced to 500.

While Cell might technically be Android 21, this character is a Capsule Corp. creation so perhaps it is just the 21st android that they created. Conversely, if the numbering process is actually inherited from Dr. Gero... well frankly I'm not sure why Capsule Corp. would even do that considering the first twenty androids weren't theirs and the man they would be taking the torch from would be kinda very evil. But he hasn't posted a history yet so no need to get too deep into judging what hasn't even been written.

EDIT: Though I will note that you don't have to break the Stats section into two power levels each. The Stats are derived from the maximum power level so there is no need to include idle stats. The power level section is only broken down into "idle" and "maximum" so that we know what scouters or people with sensing ability can detect when they are not fighting and when they are, respectively. As well it provides a min-to-max range of what their power may be at during a fight. Of course the "idle" level isn't actually a minimum as some characters can probably hide their power, but the point remains the same.