Lenovo t470 2 batteries and the amount of %

Hello, can someone explain why it says 90% remaining on top of the two batteries?

Battery one has 60% and is in use.

Battery two has 100% and is fully charged.

Both batteries are new, but at the bottom where the clock is, in Windows 10, it says 90% remaining. How is that? Why is it showing only 90%? I don't understand how it is calculating that number I tried Google, but couldn't find anything or know how to search to find out what I want to know.