So it's not really "guessing", it's just seeing if my drawing matches a predetermined bank of answers? Makes sense, since one time it asked me to draw a "police car" and before I had even finished drawing the chassis of a normal car it had already guessed "police car" and moved on.
It's probably assigning a number of possibilities with a confidence rating, and as soon as the confidence rating is > than some percentage of what it's asking you to draw, it says it.