The tank average resets when you fill up the tank, so the first few km you drive the average calculation has very little distance to calculate an average, so fuel consumption changes like accelerating or driving up hill or coasting have huge impact on the calculation. As the distance you drive increases after fill up, the greater distance used in the calculation will create more stability in the result. This is the fun part of filling up. I once had the opportunity to fill up at the gas station at a high elevation north of Toronto. I managed to put on 80km with a tank average of 4.3L/100km (almost 55mpg) because it was mainly downhill. Alas, normal driving quickly brought it back in line.

In contrast, the cruising range does not reset after each fill up.

Two climate control systems, one inside and the other at the tailpipe.

2007 Camry Hybrid (in service June 2006)
FE: 6.77L/100km (34.8 mpg) over 37750km (23450mi)
75% city - Ave trip 20 min - 2 Winters