Well, what difference does that "e" make? It is certainly not a phonetic difference because poll and pole are homophones. The "e" definitely makes a difference orthographically, however, because the difference is noticeable in print. Obviously, though, the most important difference the "e" makes, and the difference to which the writer was undoubtedly referring, is the semantic difference.
A poll tax is a fixed amount of money that a person must pay as a requirement for the right to vote.
And a pole tax is...well, you get the idea.
On a separate note, it is not just the "e" that makes the difference it is also the deletion of one of the "l's." If it were simply adding an "e" to poll then we would be talking about a chicken tax.