so then the person who only had 9 animals would not have to pay the income tax.
Tithes weren't based on the number of animals a person had, but on how many were born in a year, ie God's increase. Tithes weren't a tax based on what a person earned, but were God's portion of the increase that he gave.
As far as I can tell through scripture, the tenth newborn animal was the tithe; not 10% of 9 animals. I assume tenth animal means every tenth animal instead of just the tenth animal that was counted.