Dark Voltage

This is exactly how i feel when im driving my car. I always have my hand on the EV button so that im ready to switch between modes. I think its a habit from driving stick shift before i got this car

I love my FFE. I got it in March 2015 with a full tank and i havent been to the gas station since then. My commute to work or school is about 22 miles. So round trip is about 45 miles. The reason why i got the FFE is because i can charge my car at school and at work for free so thats an extra savings for me. The only time i have to pay for a charge is when i charge it at home. (I ONLY do value charge) I'll be hitting 1,000 miles in my odometer by tomorrow. I drive alot on the freeway with heavy traffic so this helps with the Regen. Does anyone know why my MPG (NOT EV) miles go up every once in a while? For example Last week I had 405 MPG left to drive and today i have 460 MPG. Im not complaining but im curious as to why this happens.