My inference from that is that you're saying that it just completely wigs out. If you're not saying that, I didn't mean to put words in your mouth. Either way, it's worth a clarification of what's actually going on.

When both your fingers cross over the same line parallel to one of the axes, it tends to swap the x coordinates of the fingers, but not the y coordinates. (Or the other way around.) It seems to be fairly consistent behavior.

It also has the same symptoms if you put both fingers on the screen at the same time.

I'll bet that the sensor is sending the set of x coordinates that are being touched and the set of y coordinates that are being touched, rather than tuples of each distinct finger. When one finger is put down, there is one x and one y coordinate, which is pretty straightforward to interpret. When a second finger is put down, another x and another y coordinate show up, which is, again, pretty easy to interpret. But then the axes cross and the sensor is reporting two x coordinates, but only one y coordinate. At this point, the system still knows where the fingers are. But as soon as it sees a second y coordinate show up, it doesn't know which x coordinate to associate it with. Sometimes it gets lucky and sometimes it doesn't.