While the choice of the base currency of your account is, oh, somewhat important to you, the account owner, it has no bearing on the amounts of interest credit you receive and interest debit you pay on any fx trade placed in the account. Only the interest, if any, on the account balance, is affected by that choice.
Getting back to the OP's question, overcharging on fx interest debits, underpaying on fx interest credits and underpaying interest on balance are probably a non-trivial profit center at many fx dealers. It's made possible because the process often lacks transparency, with the actual interest rates hard to find, hard to interpret (swap rates) or not posted at all. Did you ask FX Sol about this discrepancy and, if so, how did they reply?