Like any disk in macOS, Drive has a Trash. If Finder reports Drive has a certain amount of available space, but PostLab shows a different amount, empty your Trash while Drive is connected.
Using Drive with multiple Team Members? Each Team Member must also empty their Trash in Finder while Drive is connected.
Drive metrics are updated hourly. If you emptied your Trash with Drive connected and Drive Storage still doesn't match what you expect, wait an hour, then click 🏠 or quit-then-relaunch PostLab.
Empty, or not?