It's a variant of the basic human race that replaces your Ability Score Increase trait.
The "Creating a Dragonmarked Character" section says (Wayfinder's Guide to Eberron, p. 90; Eberron: Rising from the Last War, p. 37):
Dragonmarks are associated with race, depicted by a combination of
variant races and subraces.
- For humans and half-orcs, a dragonmark is a variant race that
replaces normal traits associated with those races.
- For half-elves, a dragonmark is a variant race. You keep some of the
standard half-elf traits and replace others with the traits
associated with your mark.
- For dwarves, elves, gnomes, and halflings, the dragonmark replaces
As you can see, it says that for humans, the dragonmark is a variant version of the race that replaces specified traits of that race.
And under "Mark of Making Traits" (WGtE, p. 101; E:RftLW, p. 45), it says:
If your human character has the Mark of Making, the following traits replace the human’s Ability Score Increase trait in the Player’s Handbook.
It lists an ASI for the dragonmark, and a number of other features.
It is applied to the basic human race, and is a new variant thereof; it is not intended to be applied to the variant human, which is a different variant of the human race already.
Note: D&D Beyond seems to have an error on the standalone race listings where the dragonmark subraces/variant races also appear (if you have bought the subraces/variants themselves, not just the "compendium content"). While it includes the information about the dragonmark, the associated house, some examples of characters with that dragonmark, and the associated traits, it seems to leave out the description of which traits (e.g. your ASI) it replaces, or whether it serves as your subrace (e.g. for halflings). This is only true of the standalone race listings; the compendium content itself displays the full list properly.
For instance, for the Mark of Making human, the human race listing includes some improperly formatted headers: both the Mark of Making Human heading and the House Cannith subheading (that should be a subsection of it) have
<h3> headers. There's also no specific subheading for the Mark of Making Traits themselves as there is in the PDF/compendium content (they just appear under the House Cannith section), so the text I quoted above under "Mark of Making Traits" doesn't appear there either - which may be the cause of your confusion.