One reason for the different weights could be tire sizes, If I remember right the 525 had 14.9 X 26 and 6.70 X 15 tires as standard equipment. The book shows the weight as 8340 lbs. when equiped as such.
When the combine is equiped with 16.9 X 26 and 6.70 X 15 tires, the weight is listed at 9125 lbs.
I looked up the wright of a MM 2890 (Same machine as a 525) and both list the same info.