Some pinion gears are made differently than others.
Some pinion gears are "Hobed". You can tell by looking at the involute surface of the gear teeth. If they have a wavy appearance, then those are "Hobed" gears.
Some are "Shaped" gears. They will NOT have the wavy appearance, and generally run with less tooth to tooth error, meaning they are more accurate, run truer, and will make less noise.