Why are some games valued at only 1 TF2/CS:GO key when they cost much more than that?

Those games were just part of the Humble Monthly bundle, which had, if I recall correctly, 7 games, including Rocket League, for $12

1. Possibly because of frequency of sales.
2. Cost in other countries.
3. Possibly demand.
4. Humble bundles
   Either one of them or combination of above.

when a game has been in some bundle, its value usually follows how much it costs in that bundle.
(divide the number of games inside that bundle and you will can price them equally or proportionally)

Because they really aren't.

Trade values aren't the same as "standard" values. If a game is bundled or on sale, it loses value.

Also some games have been giveaway from IndieGala, or other sites, so some sellers will still put a metal, ToD, or CSGO/TF2 key price on them that is lower than retail.

As others said, it depends on how much someone bought it for. If the person bought a 10$ bundle with 10 games, you'd expect a dollar each game.. Some people have fair prices, others not. While some people trade their Humble Monthly games for 1 key, others want 3 for example :)