I have a old Lennox Furnace from 1962?

hi, I got a lennox gf3-110 from 1962. The input BTU is 110,000 and the ouput listed is 88,000 BTU. If you use the efficency formula INPUT BTU x EFFICENCY = OUTPUT BTU, You will find that this would be a 80% efficient furnace. Now I know that a furnace from 1962 should be about 40% efficient, did Lennox mis-represent the Output BTU somehow?? According to HVAC Calculator my house should have a furnace with about 48,000 BTU. If I replace 88,000 BTU with 48,000 BTU it would make my furnace 48% efficient, which would be more where I would expect it to be. I guess my question is, did they change the way output BTU's were calculated at some point in the past or am I missing something here??