Boccaccio + Chaucer = Same Stories

One of my options for Short Paper 1 was based off this painting inspired by one of Boccaccio's stories in The Decameron. I really liked the painting, so I read the story that wasn't assigned in class. It was the fifth story on the tenth day, told by Emilia.

So then I sit down and read The Franklin's Tale in The Canterbury Tales...and low and behold...it was the same story! There were a few differences, but the basic plot of the story was the same: man loves married woman, married woman promises her body to the man if he achieves an "impossible" task, man achieves task through magician, everyone is gracious and generous in the end and everyone wins.

I was just struck by the coincidence that the one story I chose to write part of my Short Paper on was the same story we were assigned to read in Chaucer. crazy.

After I got over my initial shock, though, I started to wonder. Why was the task that Aurelius was given in The Canterbury Tales (making rocks disappear) different than in The Decameron (garden)?

• Did Boccaccio hear a different version of the tale than Chaucer, since many of their stories were part of popular culture at that time?
• Did one (or both) take creative license, and change the story for a certain reason?
• Or some other reason that I can't think of?
  

Also, I love Chaucer. I am so happy we get to read him again.