Question about dropping robots

[Puzzler]

I dropped 3 ships as robots on a world with population 21. My news reported that 21 humans indeed died. I expected, though, to see a surplus of 3 robots remaining on the planet, thus giving me ownership. Instead, the population and robot population both show as 0. Any idea why that might happen?

[Puzzler]

Never mind, figured it out.

[drwr]

For the benefit of others coming across this thread, it works out like this: dropping 3 ships means dropping 6 robots (each ship becomes 2 robots). Each robot kills 4 people, with any fractional robot rounded down. So, the first 5 robots and the first 20 people killed each other, and the 6th robot faced off against the 1 remaining person and they killed each other, leaving none left standing.

David