It's a weird thing, but I noticed the same thing.
My friends and I all have DS Lites running DSTTs using the same firmware (I kind of coordinate everything for us, so I know we're similar)
My DS is a new-ish Black/Cobalt DS bought in January
My Friend's Wife's DS is a brand new silver DS bought just this April
My Friend's DS is a refurbished white DS that was sent back from Nintendo last summer.
My friend's refurb picks up Wi-Fi faster than both the newer models. Just for the sake of being a complete moron, I tried the ol' Nintendo Blow Job trick on it, since it does help for broken L&R buttons. So I put my lips on the wi-fi light and blew... and nothing changed.
What's fun is when we have all three of them stacked in the van and are going somewhere and we all have different numbers of treasure. In this case it seems the newer model picks up the most over time, though mine sometimes does, too. That could be dependent on how close to the houses on the side of the road they are, though.
I guess some DS's pick up wi-fi faster, even if they are the same model.
Edit: Not much to it yet (just connected to an actual Wi-Fi adapter yesterday) but here's my CTW site:
http://www.clubtreasureworld.com/users/2237