Just to be sure, by polarity I mean: when the north pole of a magnet passes each one (vs a south pole), do all three switch on (grounded), or do any of them switch off (floating, 5v if connected to the controller's pullups).
If the middle sensor doesn't operate differently, there isn't a reason for it to *be* different; it costs more to use different parts both because of the labor to make sure of the different part in that place, and because they get less discount on the two separate parts quantities than if they bought more of just one, and all the handling/storing/etc of keeping them separate..