Expedition: general discussions.

But it chooses based on some resource, right?

Yes. In this case, you initially have 10 strength and, consequently, 100 attack. You can invest experience into strength, but you initially have 1500 experience, and it takes at least 100 experience to get +1 stat (the more you level up, the more it costs).
If you're lucky with an artifact, your attack might increase, but the artifact depends on the GM's random selection.