Tires Exact same tractors--two different tire sizes?

I recently aquired a second LT160H42CBV2 [LX1642H] and at the time I thought it to be the same tractor as the one I already owned and had bought new in 1998. However, while they both have the same rear wheel size, they have two different tire sizes. One has 20x10.00-8 and the other has 18x9.50-8. All the tires appear to be the original aged Monitor brand. What is to be gained by having two different rear tire sizes? I believe I am baffled. Thank you for any and all responses.