Gas Fill Ups Amount Vary

I usually get gas at about the 1/2 way point on the gas gauge. A couple of fill ups ago I only took about 4.4 gallons. And I was pleasantly surprised! The next time I filled up, at about 1/2 tank, and I took 6.6 gallons!

Is taking only a small amout of gas at 1/2 tank (4.4 gallons) caused by the bladder in the tank being expanded? And what is the purpose of the bladder?

Is it better to just rely on the mpg readout on the consumption screen or calculate the mpg by dividing the miles by gallons pumped? Is the consumption screen pretty accurate?

Thanks! Marvin