Story Points measure a combination of Complexity and Effort, which is what makes them hard for people to understand. Risk, uncertainty are likely also components that are part of the estimate.
In the end velocity is a measure of what you have delivered in the past (few) sprint(s). It's an indicator of what you can expect to deliver next sprint when nothing changes much.
We do see that Scrum teams improve over time however, they get better at what they do, they automate things that take up a lot of time each sprint, they increase their knowledge of the domain and they start to be able to re-use code elements from previous sprints to build on top of.
As such it's normal to see teams increase their velocity over time. Often that velocity reaches a ceiling, their maximum capability given the current circumstances, and won't be able to start climbing again without a major event. That event could be training, changes to the way of working, a new team member with a key skillset or a switch from one technology to another (or simply an upgrade from vold to vnew).
Most teams pick a couple of reference stories. Some just pick their 1, 5 or 8. Other teams pick a 5 and a 20. it doesn't really matter what they pick. Given these reference points you compare the other stories to the original request. As such, the number of story points between two stories is often equal, even though the exact amount of effort has reduced drastically over time. This is why a team may be able to increase its velocity from 40 to 200 over time.
When a team doesn't feel they have enough granularity in the lower reaches of their story points, they may opt to pick a new 5 and adjust all the other items on the backlog accordingly.
Remember that increasing velocity is not something which we should try to encourage all the time, treating the velocity as a metric will not help the teams understanding of what they can actually do and usually leads to bad behavior when it comes to estimating. Instead, as a scrum master you should focus on removing waste and increasing value delivered per sprint. If the team has the same focus, their velocity will rise as a result.