I'm pretty sure it's intentional, and in the source we can see that the mechanics are the same for wood and furs. Both mysterious wanderers will accept 100 units and have a 50% chance of returning with 300. Average marginal return per event: +50 units. You learn over time that this is generally a good deal, and you begin to trust the NPC.

Eventually, the player may get greedy. You'll try giving a mysterious wanderer 500 units of a resource in order to speed up the process, under the assumption that the odds are the same. At this point, there is a 70% chance that the wanderer will be tempted to abscond with your (relatively large) investment. Average marginal return per event? −50 units.

And that's why "builder thinks he isn't to be trusted"!

I played the game through and never realised the wanderers could be trusted at all - I gave them resources a few times, but never got a return!

Oh, I see. I misread the "100wood" one. You're right.

It would have been a neat twist if one of the traders was more trustworthy than the other.

I almost wish I hadn't read the source ;D

This seems like a good, small modification to make and send in a pull request for.