You should take note that not all of their clients' trading history are being submitted to the IRS for review, the order for them was to submit "random" trade reports or unusual amounts of trades coming from their clients. And yes being a U.S. compliant exchanges I expect them to be one of the companies being required to submit trade reports.
poloniex has said repeatedly that they do not do
any tax reporting. i'm sure if the USA government sends them a subpoena for your data, they will comply, but that is a very rare situation. in the regular course of business, they do not send 1099s to the IRS like coinbase does.
@OP are you sure that your calculations are done properly? Because the IRS won't even be on your radar if the gap from your real earnings is that big from what you have reported.
indeed, it sounds like the OP received a 6173. from other reports, that indicates he was cashing out through coinbase and not fully reporting the taxes due.
if it were me, i would
at least run all my coinbase data by a CPA and probably plan to amend schedule D and form 8949 to match what the IRS already has.