No other reason except that some bows just tune that way. It may have something to do with the fact that the arrow is launched about 1 1/2" above the bow's vertical center. Might have to do with how the bow is gripped or not gripped, as the case may be. There might be very slight difference between the upper and lower limb.
If a launcher style rest is used it may have to do with how much spring pressure is on the rest. Might be the stiffness of the arrows used.
As you can see, there may be many reasons.
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