121.391
121.391 (a)
(1)
"For airplanes having a maximum payload capacity of more than 7,500 pounds and having more a seating capacity of more than 9 but less than 51 passengers -- one flight attendant."
(2)
"For airplanes having a maximum payload capacity of more than 7,500 pounds and having more a seating capacity of more than 19 but less than 51 passengers -- one flight attendant."
My guess is then that those aircraft flying 121 ops with 19 or less seats have a maximum payload capacity of less than 7,500 pounds. Even if the manufacturer says the aircraft can lift more the operator can reduce the capacity through the OPSPECS in order to comply with the regulation. The SD3-60s that I used to fly in cargo service were limited to 7,495 pounds in order to comply with part 135. Shorts listed the payload at 8,300 pounds.
Hope this helps.