Because you indicated that you sold the home early due to a qualifying "unforeseen circumstance" (like a change in job, unemployment, change in health that required you to move, or a few other things). You are then eligible for a partial exclusion -- in your case, you lived there 75% of 2 years so you qualify for 75% of the normal exclusion of $500,000 for married filing jointly.
(I'm surprised it's exactly $375,000 since the exclusion amount is figured to the day. You must have owned the home for exactly 548 days -- 75% of 731 days.)
