Z-Flex 4" x 40' Aluminum Liner Kit v600 Why is the BTU according

Why is the BTU according to EPA test data smaller than the one advertised

10’ Length

Fit PyroMania Monument

30" Round Tile tabletop

Z-Flex 4" x 40' Aluminum Liner Kit v600 Why is the BTU accordingUL listed aluminum chimney liner kit for use with Category I natural draft gas fired heating equipment and gas fireplaces. Note: For Use with Natural Draft Appliances. Not intended for use with Category II, III, or IV gas burning appliances as defined by the National Fuel Gas Code, NFPA 54, or other appliances that result in condensation or corrosive acids on the liner of the chimney, or that create positive pressures in the chimney system.