On Machusetts highways, why do people p on the right instead of the left when both options are available?

Whenever I'm in the middle lane, I notice more cars p me on the right instead of the left. Why is this? I can understand if there's a car in the left lane, too, and they use the right lane to get around both of us. But often, there is no car in close proximity in the left OR right lane, yet the driver chooses to p me in the right lane. What gives? In NY, and most interstate highways, the left lane is the ping lane!