Because of the possibility that there are multiple sellers in the same zip code. Certain cities (City of Industry, California comes to mind) have hundreds of merchants who sell online which aren't affiliated with each other in any way.
That is what I thought. On Walmart though they will ding your metrics if you ship confirm using a tracking number that did not “originate” with the zip code on your account...
Same shipping zip code is only 1 similarity and usually not enough to link accounts
But if you wree shipping from the same zip code, and selling the same items and had the same routing number at your bank, that may be enough to link/suspend