If its a wide region ad (can even be just across a metro area) showcasing a price then yes, they wouldn't know the price at a given store because the tax rates can change in less than a kilometer.

If there's a TV ad for a medium pizza for $10 at a chain they can't possibly know the tax rates for whatever actual store I'm going to go order from.

And the listing on a website won't know until I actually put in my shipping information.