To get an accurate reading on your mileage, you need to visually fill the tank to the (exact) same level each time, or check the mpg over a few thousand miles keeping a log of how many gallons are used. If you are not keeping track of mileage over the long term, make sure that you fill up at the same location (so the lean angle is the same) and visually fill the tank to the same level - be careful not to overfill, the bottom of the fill neck is where you want the level to end up, not above.
With the small tank on MCs (and the Savage has one of the smallest, a small difference between one fill up and the next can make a significant difference, I don't think you said how many miles you were basing your calculation on, but if you are going about 100 miles between fillups, which is pretty typical, you wouldn't be able to fit enough gas in the tank.
If I happened to leave the choke on my way to work when I rode the Savage, the engine would be completely warmed up and not running right once I pulled up to a stop sign (reminding me that I should push that thing off). I never noticed much of a difference in mileage, but I'm sure there was some.
Even with only 4000 miles, your Savage is close to 7 years old, with one oil change - I'd get it changed
now