Don't you think the fact that Finex's 4 Taiwanese banks have frozen transfers (under pressure from Wells Fargo) has something to do with it?
I find it rather odd that this question is still popping up. The reasons are on Bitfinex's site and it's obvious why it is how it is.
it's not this simple, perhaps.
BTW, bitfinex uses USD or USDT?
btc in USDT is also abnormally higher on polo than on GDAX or coinbase (which use USD, of course).
maybe it is also related to tether, which never liked anyway.