Why does my battery percentage decrease if I drop my phone?
Dropping your phone shouldn't decrease your battery percentage.
I should know, I've done it on more occasions than I can remember.
However, if the drop is sufficient enough, it could cause internal damage to the battery which is actually a very dangerous occurrence.
Batteries are Lithium Ion & they shouldn't be pierced due to their significant threats as fire hazards.
If you're concerned then I recommend taking your phone to the manufacturers store to be looked at further.