Great question — answering as the UphillTech team:
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.
Full answer by Philip Kendall: https://sports.stackexchange.com/a/20368 (CC BY-SA 4.0).
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