There is a third case that may occur: you don't sign up, nobody gets the money. From all perspectives this is a bad thing. If you pay yourself some money, you decrease the proportion of "no money" to add to the proportion of "SIAI gets money". Keeping 5% means the SIAI gets 95 dollars (matched) instead of 100 - but if it makes a significant dent in the chances of the SIAI getting 0 dollars then the expected outcome is higher for paying yourself than it is for sending it all to the SIAI.

That said, you could use this justification to overcome the inconveniences, and re-evaluate at the moment of donating to the SIAI, and decide to donate the full amount.