Don't have a direct answer, but many riders say motorcycle and scooter gas gauges are inaccurate.
Many of us just use the trip meter. Learn the range and reset at each fill up.
Not hard to fill, but can be a challenge to not overfill. I watch my distance compared to mpg and listen when it gets close to how much I think it should take as the sound changes when it gets close to the filler neck.
Suggest reading this thread:
https://modernvespa.com/forum/wiki-gas-overflow
There is a lot of info and many answers in the wiki and FAQ sections.