There are two types of power ups.
If you have enabled Twin Power for the ship you currently have (you have to unlock Twin Power individually for each ship, normally obtained during Events or PvP Seasons) then instead of a single power drop, it will randomly drop a Twin Power. This will increase your current bullet level by two, up to the maximum allowed for your ship. The chance of this happening increases slightly the higher you have upgraded the bullet level of your ship. Twin Power occurs in roughly 1 in 4 power drops, when unlocked.