Not seeing the margin compound

Each leg of a parlay carries its own overround, and combining them multiplies the prices — so the margins multiply too. Four legs at −110 give away roughly four times the fee of a single bet.

Multiplication is what makes parlays profitable for the house. A single bet gives away about 4.8 points of probability; each additional leg applies its own fee to a payout that has already had the previous fees removed.

This does not make every parlay a mistake. A small stake on a long price is a legitimate purchase of entertainment, and a correlated combination the book has mispriced can be genuine value.

Here is the catch. The payout is the headline and the compounding is not. A menu that shows '12/1' is showing the half of the arithmetic that sells the product.

What to do with it. Multiply the legs yourself before accepting the offered price, and treat the difference as the cost of convenience.