If a team wins by 30, the total is very likely over. Betting that team's ML + over is correlated — when one wins, the other is more likely to. Books either disallow obvious correlations (same game ML + spread) or reprice via "same-game parlay" products with uncorrelated pricing.
Advanced concepts
Correlated parlay
A parlay where the outcomes are statistically linked — books usually block or reprice these.
Example
Chiefs -10 ML + game total over 50 are correlated. Most books force you to book these via SGP, which removes the correlation edge.