Sure you only weighed in at around 105 when you started? (or were you shorter then or did you maybe measure in kg or stone and didn't convert right?).
I mean, I started at 120 and that was anorexic at 5'10...
As for putting on 100 lb in a year... Well, if you're underweight to begin with, it's possible.
Not all muscle, but you are basically going from completely detrained/stickman to average-sized.
I see quite a few guys go from ~120-150 at average height (say. 5'9 to 5'11) to 180-200 in a year without gaining much fat. That's pretty much the newbie gains you get once you start eating a little more and a little more often + getting stronger practically every session. 50-80 lbs gained overall, normally.
I don't have much experience when it comes to taller guys, but I guess another 20 lbs due to the larger frame may be possible...
A guy who starts underweight can put on a lot more weight with relatively little fat gain in a year than a guy who starts out average or above average.
Still, starting weight seems very very low for your height. 105 lbs at 6' is practically concentration-camp-victim size.
Anyway, good job. Things are going to become a lot more difficult soon, so good luck.