The meter dial is a geared device. If the flow meter is moving forward it adds to the water usage, if it move backwards it subtracts from it. Bouncing back and forth is caused by pressure changes in the water main. When it drops, (The main may be undersized so that usage in your neighborhood causes it. Similar to opening a faucet while someone is taking a shower.), the pressure in the house causes the dial to "bump" backwards momentarily until the pressure equalizes.
in 2006, I installed an Super Stor 45 gallon indirect heater tank. The T&P valve I installed on it kept relieving itself every so often before I installed a Potable water "XTrol" tank. Then, I noticed the water meter doing the same thing, running forwards and backwards as the pressure in the mains outside varied as observed on the little flow indicator on the water meter. I don't know if it ever caused my water bill to be increased, but I am pretty sure that the 1/10 Cu Ft dial would go forwards and increase, but never go backwards when the flow indicator ran reverse. Sometimes the mains pressure would vary with enough frequency that the setup seemed like a ratchet device in the meter. I added a check valve in the cold water feed to the tank, and that stopped it. Haven't had a problem since. The water meter was installed in 1975 or so if that makes any diffenrence in how it was built back then.