Good question.
The honest answer is that we just don't know, however, here are my thoughts on the subject.
I believe that some of the short arrows were found in a bundle with some of the longer ones, now if they were for different archers, which is the obvious answer to the different lengths then why weren't they separate?
Now we have fairly firm evidence that to reach the distances attributed to the English archers they would have to have used a different style to the modern archer and "over-draw" to beyond any anchor point. This would explain the longer arrows, however, it is much easier to shoot accurately with an anchor point of some description, and if you were to do this with an arrow that is too long you will end up with the arrow falling short due to the excessive weight of the shaft. So you would want an arrow the "correct length" when doing this.
I believe (Although I admit I have no proof) that the archers may well have carried some shorter "hunting" length arrows that were matched to they're size and bows to shoot when the enemy got too close for comfort and you wanted to take aim and know that you would hit the guy that you were aiming at, where you were aiming, and not just take out one man in a crowd in the distance.
I don't know this to be the case, but I don't believe that they would draw a point inside the bow, because of the damage that this could cause. (particularly without an arrow plate!)
Daniel
(PS don't try over drawing a
longbow that's not been made for it. It'll break!)