OEM and aftermarket speedometer gear difference

Just swapped the 33-tooth oem gear in my torqueflite for a 31-tooth and they looked totally different! The 31-tooth was considerably smaller in diameter and had a totally different tooth design.

I got it indexed properly and took it around the block, appears to work as advertised but I am confused how only 2 less teeth equates to a gear that is considerably smaller. Shoulda took a picture. It really makes me uneasy that this one is going to eat itself or something.

Seller was 'transpros' on ebay, they claim to offer exchange after 30 days, anyone have experience with these? Am I worrying about nothing?

Because of the tooth pitch, they had to change the diameter of the pinion a few times to match the pitch of the teeth on the drive gear. IOW, if they had kept the same diameter for all tooth counts, the larger tooth counts would have tooth pitch too small to match the drive gear.