How does the Parlays tool work out the EV of a parlay?

NinjaBet

It combines the legs the same way a sportsbook does, then compares the result with the combined fair odds.

Each leg is converted to decimal, the decimals are multiplied and the total is converted back to American. For example, +120 (2.20) and −110 (1.91) give 4.20, which is +320, so a $10 parlay returns $42.

That $32 is the potential profit if every leg wins. The EV is a different thing: it tells you whether the combined price beats the combined fair odds. If each leg has value, the parlay has value too.