"About this mac -> Storage" show different values than "Macintosh HD"
These show absolute TOTAL space, not usable space. For instance, you’ll have some space used for swapping. Clearly, swap is part of the total amount, but not available for use for general storage. Also, depending on how the disk is partitioned that will affect space. For a more practical display of space, try:

df -h


For instance, those tools show my disk space as 4GB in total, but that’s not all available for my use: df -h shows it as 3.6 GB.