Piro speaks:
Well, I suppose it's not very clear to see, but Piro has never really taken a good look at them. Piro has a tendency to avoid eye contact (especially with agressive girls) - and he hasn't seen her enough to identify her with a very different hairstyle. And when he was following her around, i don't think he spent a lot of time looking at her face - the pigtails were the main identifying characteristic.
But the main thing would be - note the differrence between how she agressively confronted him initially, and then the fact that he never DID really confront her or deal with her when he was following her (trust me, he has no clue why she ran away), and then the far more polite, nervous way she returned his stuff to him - people are identified as much by thier behavior as thier looks. It's part of why with super-hero genre characters that do little more than change clothing, they still seem to be able to maintain thier secret identities - people don't connect the behavior with the person they know, therefore it must be someone else.
There's a lot going on under the surface of all this. When you do story stuff, its good to work in layers - there's the basic layer that everyone can understand without digging into it, and then there are various sub-themes that flow beneath the surface, that are there for the finding, if you look close enough.
Sometimes, someone points out something i never realized before, a point or concept that i didn't really make the connection with. In fact, i've capitalized on some of them, because they were too good to ignore.
anyways, comic came out better than i expected today.

See more of Piro's sketchbook here.