I've made an item in the admin console that is mapped to a tax code which as a tax rule attached to it making it exempt. It is still calculating tax.
Please be sure that even extra spaces are accounted for in your item from the ERP as compared to the item in the admin console.
A single integer and even space of difference between an item in your ERP and in your admin console will make it not work correctly.
Item from ERP had an extra space, example
ERP: 1394 A
admin console: 1394 A