Is this unusual, that he'd be able to do the computation without realizing how or why?
Unusual for ND kids maybe...unusual for our GT kids...probably not. I know that my DS6 can do a lot more computation than he really fully understands. He loves fractions as well and the patterns that he finds in them. He adds, subtracts, and simplifies them but I am not 100% sure he could answer all the why questions.
BBDad:
You descriptions and time lines of your DS sound very familiar to my DS6. I wish I had kept records of his math milestones but I didn't realize how unusual it was until he closer to 4 years old. By that time it would have pretty hard to go back and figure it all out down to the month.
My DS6 also knew all his letters early but he didn't actual read whole words until around his 4th birthday. Within a year or so he went from very early beginning books to chapter books. I would say he can read almost anything right now. DS6 never seemed to learn phonics. When he comes across a new word he asks me what it is and then he knows it from that point forward. Surprisingly though he is becoming a pretty good speller. Even though reading has come so easy DS6 he still does not read for pleasure. When given a choice he prefers math.
OP: I also think that you can't discount math talent just because it comes later. According to the standards of other children on this board my DS6 is a late reader but the rate of his progression has shown that he does have a talent for it. Also just because he has a talent doesn't mean that he will ever have a passion for it.
