Not to be argumentative, but it is not that Quicken downloads those transactions, it is that IB provides those transactions to Quicken. Why do they do that? What do they show on their website for your account?
What is the real-world meaning to the change? Is it a change in value (a price change), or some level of actual transactions. The MiscInc or MiscExp transaction would suggest a change in value of the cash in your account. Is that accurate (real world)?
With exceptions, using hidden categories (_xxxx) should not be a common practice. IMO, assigning _UnrlzdGain to a 'real world' transaction would be even less likely to be the correct direction. As I understand it, Quicken only uses _UnrlzdGain when it need to present a change in value based on a change in the price (the markets valuation) of the security.