XP and Char Advancement

Under Char Advancement in the SRD, it talks about giving 1-4xp for various reasons. The fallout book says 20-40xp per adventure. And yet, in Conan it suggests about 300xp per adventure.
Fallout, levels restrict Perks.

I am confused by all the variations of XP.

The XP values in different 2d20 RPGs are on different scales.
The old ones, MC3, Infinity, Conan, use the XP scale in the hundreds, usually you get 100 XP per hour of play, in Conan half that, so 50 XP per hour.
A!C has much lower numbers.
Others, like STA, don’t have any XP at all.

In the end the “buying power” per XP is differently scaled, too, so that you get about the same increase in character capabilities per session or per adventure.
There is a difference, though, as in the old 2d20 games the Talents are usually less powerful at lower ranks, and so you will have to buy more of them to get to the more powerful ones at higher tiers in the talent tree. Other 2d20 RPGs don’t use this tree approach, so you can learn those talents often right away.

Fallout is the odd one - it is a strict level-based system, so things are quite different there.