I am able to see one of my own giveaways (Solved: it's because I don't own that game)

No idea if its the reason here, but an easy answer would be that he's got no intention/time/mood to play the game at the moment and a big backlog so it would only be collecting dust in the library. :)
Have given some wishlisted games away myself because of these reasons.
Not everything that is on peoples wishlists is a game you'd play the second you get it.

Why do you give away games you have wishlisted but don't own yourself?

that happens more than it may seems. there's some true generosity in that way of giving, reasons could be different, or multiple.

maybe you want to support a dev, or might be just that... "play it, it's yours, i think you'll enjoy this"

kinda "true" giveaway.

edit: nice question, btw

I don't own this game and I am still interested in it enough to keep it on my wishlist. I am a member of a group called JediTraining and they require you to add a giveaway to every train you win from with a couple exceptions. I could have found a cheaper game to match the criteria 5P NON-Bundled Train but I want to show my appreciation for the group since I have been so fortunate there :]

I'm currently giving away a wishlisted game too, if I buy something to give away I look for a game a lot have wishlisted so it may happen that it's on my own wishlist as well. I have a huge backlog anyway and I know I'll still get the game for myself eventually so it's ok for me.

I sometimes buy like 2-3 copies of a game I've wishlisted but haven't added it onto my steam library yet. With making giveaways, I keep an extra key back sometimes in case of dupes or issues with it as I personally don't like making giveaways I don't have a spare key for usually. Humble bundle keys I am wary of especially as they consider it trading on SG so I like having a backup.

This game hasn't been removed from Steam so that's not the problem here, but that's good to know about for the future!

I did try to sync my profile but it didn't seem to make a difference. It looks like a side effect of giving away games that aren't in my library, I guess I never noticed because I don't do that very often :D

I don't give many games I don't already own, so I hadn't noticed this before! Thank you for clearing this up :]