If you want to really focus on how and why people are "improving faster" than you, the answer is simpler than it seems.
They just play more.
While everything everyone said is true and would help with improvement, at its core, improvement is heavily tied to how often/much you play.
With 700-900 monthly plays, you're certainly playing enough to improve, but its always going to be slower than someone who has 2000+ monthly plays just from the sheer fact that they're playing twice as much as you.
This shouldn't discourage you, but you shouldn't compare yourself to others with this in mind (or in general, really).
One other factor that also should be taken into account:
It looks like you started essentially brand new to VSRGs, and appropriately started at SR1.
Many people who come to Mania often do so after having spent months in other games (Etterna, Robeats, FnF).
Since they have prior experience, they often start somewhere between 2*-4*, which makes them seem like they're improving faster than they actually are.
In reality, they just spent their first steps somewhere else.