A QB's passing-yards over and his top receiver's receiving-yards over win together far more often than independence would suggest, so a book pays an SGP containing both much less than the naive parlay multiplication. The gap between the naive product and the offered SGP price is the book's correlation haircut - sometimes fair, sometimes very much not, which is why comparing SGP prices to independent leg prices is revealing.
ParlayAPI's SGP demo shows correlated pricing on live games. The building blocks - per-leg prices for game lines and player props from many books - come from the standard odds endpoints, so you can compute your own naive-product baseline and measure any book's correlation adjustment.