2 different dynos,2 very different numbers.why?

About 2 years ago, someone PM me about a 360 that he made a deal on with an engine shop that recently closed up. X amount of dollars for a 550 Hp stock stroke 360. The dyno sheet he got only showed just over 500 hp. He asked for a refund of x amount dollars since he did not get what he paid for. Several days later he got a new dyno sheet in the mail showing the engine made 545 hp with the comment that they had re calibrated the dyno and that no refund would be coming.....Should have saved those PMs.....