This might be the difference of GiB and GB, meaning GibiByte and GigaByte.
But this should result in round about 1 GB Difference.
Someday they changed the measures. I long believed that 1 GB has 1024 MB. Once true (not sure however to when, 1996?), this today is wrong, for quite some time now. So 1 GB has 1000 MB. But 1 GiB has 1024 MiB.
Sometimes it is a mess what is shown, GiB or GB regularly changes depending on what tool is being used. Adds to the confusion.