Probably not as deep of a reason as you might suspect. Possibly one tire busted and they used a different tire they already had laying around.

That being said, a lot/most riders have different tires for their front and rear, often having deep center tread patterns on the rear for climbing, and deep edge tread on the front for cornering. Maybe the previous owners favorite tread pattern only came in a different width tire..

Lastly, difference between 2.25 and 2.2 is... ...small