Can the provisioned storage size of an RDS decrease if auto scaling is enabled?

I made the RDS with t4g.small instance and 100GiB of general purpose SSD(gp2)

single AZ, no Proxy but enabled storage auto scaling

After testing for 133 hours, I reviewed the billed charges, but there was a part that I couldn't understand.

why does the charge for storage usage is recorded for 17.655GB-Mo, I thought it should have been 100GB-Mo that I provisioned at the very first

of course it's thanks for me that they didn't charge the whole provisioned storage

but it makes me confused.