Yes - your math assumes random distribution, which isn't the case. The 767 will preferentially fill up window and aisle seats, so OP would be left with only middle seats available. This is especially true because the ratio of total plane capacity to C+ seats is way worse than the 75S (which is 26% C+ seats). While I know they mostly run 763s on this route, the extreme example of 764 is only 11% C+. Therefore, even though there are more middle seats on the 75S, there is a much higher chance there are window/aisle seats available for upgrade on a 75S than a 767.