A possible answer to this parable can be found by looking at two passages.
Matthew 19:27-30: "Peter answered him, “We have left everything to follow you! What then will there be for us?” Jesus said to them, “Truly I tell you, at the renewal of all things, when the Son of Man sits on his glorious throne, you who have followed me will also sit on twelve thrones, judging the twelve tribes of Israel. And everyone who has left houses or brothers or sisters or father or mother or wife or children or fields for my sake will receive a hundred times as much and will inherit eternal life. But many who are first will be last, and many who are last will be first." (NIV)
One thing to note is that it says "many" and not "all".
Revelation 20:4-6: "I saw thrones on which were seated those who had been given authority to judge. And I saw the souls of those who had been beheaded because of their testimony about Jesus and because of the word of God. They had not worshiped the beast or its image and had not received its mark on their foreheads or their hands. They came to life and reigned with Christ a thousand years. (The rest of the dead did not come to life until the thousand years were ended.) This is the first resurrection. Blessed and holy are those who share in the first resurrection. The second death has no power over them, but they will be priests of God and of Christ and will reign with him for a thousand years." (NIV)
I don't proclaim to understand completely the idea of a first and second resurrection. But this suggests at the idea of those who, especially in the end times, endured the torment and death for their faith, such as from not succumbing to the mark of the beast, being in the first resurrection. The rest that died by other means, mostly before the time of the beast, are of the second ressurection.
We all gain the gift of eternal life (the denarius). Those that waited all day to be hired, they hadn't given up and just gone home, still get the denarius and get paid first. They endured standing around for hours waiting to be hired knowing that normally by having to wait they'd be paid less if they got hired at all. Hardly a simple and easy thing to do. In the same way, those that endure torture and death because of the strength of their faith and testimony, notably in the end times, are part of the first resurrection. Same pay, just different times.