This goes all the way back to 1970's BMX days, maybe before ---- was customary to run a 1.75" tire in the back and a 2.125 in the front ---- the back tire needs great acceleration and the speedier smaller tire is ideal since the front is most crucial for handling --- with a bigger contact patch

-- I'm planning on going 1.9/2.1 with my new build as well for similar reasons , --- but then again , the answer could be as simple as everyone else said -- its a 6 year old bike, maybe the PO just used a tire he had laying around