For an NBA game, I see a market for Spread and one for Winning Margin. The Spread market lists "Team A -5.5: 10/11". The Winning Margin markets lists "Team A, 6 points or more: 3/4". Shouldn't the odds for these two best be equal?
There's no fundamental reason they should be equal, because a bookies' job isn't to accurately reflect the chance of a team winning: it's to make as much money as possible for the bookie. If a bookie has taken too much money on one side of a line, they'll move the odds and/or the line to encourage punters to bet the other side of the line; bookies really don't like a situation where they're at risk of taking a significant loss if one result or another happens - their desired situation is if they have equal exposure on either result, and then they're guaranteed a nice profit thanks to their cut.
They should be equal if odds were fair and if both markets apply to the same principle with respect to overtime rules. Usually handicap markets include overtime while margin of victory not (you may find a selection of draw there). Additionally bookmakers apply margin to odds, so there might be a case where handicap is offered in x margin and winning margin in y.