As the price of the stock goes up the number of shares that the preferred receive decreases when they hit the cap the number of shares stops decreasing. The shares don't disappear they just do not go to the AAMRQ shareholders above the cap they stay with the preferred.
The shares in excess of the cap is for calculating the distributions to the AAMRQ shareholders not the preferred. The base of the formula for calculating shares to AAMRQ shareholders does not include a cap and the part about "less the shares in excess of the cap" in the AAMRQ formula is where they include the cap in the AAMRQ distribution.