there are a whole range of possibilities here, from 'he went to Nepal specially to shear a goat and knit it himself' to 'he found it in a gutter and thought of you'. Okay, those are extremes, but the medium-range possibillities aren't much more help.
He went looking for a gift, saw a mag, bought it and gave you the scarf?
He wasn't looking for a gift but saw a mag, bought it and gave you the scarf?
He didn't even buy the mag, someone gave it to him or just gave him the scarf?
Or there wasn't a mag at all but he went looking for a scarf, found it and bought it/ he saw a scarf quite by chance and bought it/someone gave it to him and he gave it you???
It's going to be almost impossible to find out what really happened - the simplest thing would be to ask him, but it seems you aren't quite ready to believe what he says.
So I think you may be beating yourself up over nothing, because this is a question you'll never get an answer to. One thing I would hang on to, though, is this: wherever he got it, whatever he paid for it (or not), he did think of you and he did give it to you. Maybe you should be giving him credit for that?
If you can't do that (and I'm not saying it's your fault) then it looks like your relationship may be failing because of basic lack of trust. If you find yourself unable to believe what he says, should you be sticking with him?