Hi MalcolmAbarintos
I'm Louie, a Microsoft Community member and a fellow Windows 10 user like you. Hope you're doing well and safe there.
One possible cause of the incorrect storage size value displayed is due to corrupted system files. To repair all corrupted and missing files, I suggest running SFC/DISM.
To perform this step follow the instructions provided on the link below:
https://answers.microsoft.com/en-us/windows/wik...
Hope this helps.
Kind regards,
Louie