The move Sky Drop uses 2PP per move instead of 1, but it seems to only do this when it actually knocks out a pokemon, after trying it against a sturdy Graveler where the PP remaining decreased from 6 to 5 after taking it down to 1HP, and from 5 to 3 after actually knocking it out. This was also tried against other, non sturdy pokemon, who all took 2PP from the count despite actually using Sky Drop once to knock them out.

Thanks for the report. I was able to confirm the issue, and have passed it along. We'll get this ticket updated when we know more.