It all seems very unfair. Whether you’ve spent 12 hours or 1 slaving away to bring the harvest home, you still get paid exactly the same: the day’s rate for a working man.
Jesus’ story paints a familiar picture to his listeners (“the disciples” [19: 23]). It’s harvest time and the grapes needed to be gathered in before they spoil. The sting in the tail is an illustration of grace: it is unexpected, undeserved but very welcome.
God is not controlled by our rules or laws but works as He wills, according to his character. It really is grace and generosity combined.