This is a very good question.I looked at the forums for Saturday June 15 and I saw that for the first post and the first ticket the player edge was +6%. I looked at the website and most of the player edges for the games were negative. So my question is how do you get a positive number for that?
First let's dispense with the obvious.
The edges are based on the probabilities derived from the vegas/offshore betting line. This of course changes throughout the day. So, it is quite possible that when the original post with the parlay was made, that the edges for the individual games were different than what you are now comparing them to.
Second possible explanation,
I'm assuming that the parlay in question was a pointspread play for this answer. (It is also possible to get +ev parlays from -ev plays in proline but that's another matter again)
What you probably saw was a bunch of slightly -ev plays and a 5 (or 7) team parlay that was +ev.
The edges listed for pointspread plays are based on a combination of the win probability (obviously) and an OLG payout value of 1.78. But in reality pointspread parlays do not have a 1.78 payout. They pay 5-1, 10-1, etc. I have used the 1.78 (this is accurate assuming a 4 team parlay) as an 'working number' for the purposes of displaying the edges. But the OLG per game payout varies, and as a result the edge, depending on the size of the parlay.
To accurately determine the edge for pointspread parlays you can,
Manually - calculate the parlay's win probability by using the win probabilities of the individual games and then calculate the parlays expected value for that probability and the parlay's payout.
Or - the ProlineParlayMaker program available on my site can do this for you automatically. That would have been how the parlay ev was calculated in the post you reference.
Hope this helps,
PLP