Moros - you might note the card is in 4 pieces - that is because in a risk game you have to trade a 'set' of cards out of a maximum of 5 - and there is no guarantee you can trade in while you have 3 cards - nor is there a guarantee you will have to wait until 5. So 4 is a medium number.
For the first card you are right. However, after you trade in a few cards, the average time between trading in cards goes to three turns. The way to mimic this is to have players start with -1 card pieces (and three pieces per card), but WL cannot model that.
WL doesn't keep track of how many cards you have, it actually tracks each card; if you look at a log from the Game Feed API, you'll see each card has a unique ID number. If it simply tracked the amount of card pieces it would be easy to allow negative numbers; with this system, I don't think it can be done.
Also, if memory serves, three infantry cards are worth 4 armies, three cavalry cards are worth 6 armies, three artillery cards are worth 8 armies and only a mixed set of one of each is worth 10 armies. I didn't do the actual calculation, but sometimes you get to give cards without having a mixed set, so the reinforcement card should really be only worth 9 armies or so.