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The difference is that Momentum Change is a triggered ability and both activate at the same time so they each refuse to allow themselves the privilege of returning to your hand. Highway, on the other hand, is an instantaneous SA so by the time the second one is played the first one's SA exempting itself from return is no longer active.It is worth noting that most of the truly nasty play-for-an-hour combos require some form of this type of recursion. If this class of enhancements were specifically defined to exclude themselves from return, there would be little exigent need for the hand-limit ruling.
Hopefully with some immunity blocking in a brigade your opponents not using