A stupid mother question

I have a question if anyone is out there. What does the '2' in a toddler size 2T mean? Does it mean anything? Is it just one less than a 3T? Or does it refer to age (as I had originally thought)?

My son is over the 100% place on all the growth charts and almost 3 years old (will be 3 in December) yet the 3T pants are too long while the 2T are about right.

What's up with that? Why am I such a complete moron that I don't know this? Are all the doctor's charts wrong? Do clothes manufacturers know ANYTHING??

Or am I just a STUPID MOTHER??

WTF???!?@?!?