On each wave there is a minimum required firepower level, for example, on a max difficulty Superstar Hero Boss Rush, the required firepower level is 9.
If you have your firepower lower than that, the game will spawn as many of them as necessary so you reach that exact required firepower.
Check this out: Apple Core Charity Bug - YouTube
The “charity” bonus appears in the top left,but…powerups are nowhere to seen. Maybe the powerups spawn too far on the right sometimes,which causes them to despawn or something?