I bought used 767-300 ER. It should be able to transport 290 passengers (according to museum), but after buying it I see that it's capacity is 217 passengers. Why is that?

I believe it has something to do with 2 rows in first class. I am however unable to convert them to 2nd class. Plus and minus buttons do not work. Any ideas?

Today I noticed that is was possible to remove all 1st class seats. The total capacity was 289 however, not as advertised 290 passengers.