Hello, this is called an anticipator. Think of it as a little electric blanket for the thermostat. The anticipator either lets the stat run cooler or warmer which results in different run times.
You see the thermostat does not know how big of a furnace it is hooked to. So if you take a reading of the gas valve drae you can move the pointer to the appopiate number to match.
If you didnt have this the and the thermostat was set for 70....it would leav e the gas valve on until it was 70 degrees at the thermostat, then the valve turns off, but the blower still has to get all theheat out of the furnace. So really we overshoot 70 by the time the heat is cleared out.
So the "anticipator" anticipates when to turn off the gas valve by either warming or not warming the stat, so that by the time the furnace heat is purged you should be close to temp.
When the arrow says "longer cycle" it turns the electric blanket down thus letting the furnace run longer. Sorry to ramble so much.
Since you have the older style thermostat instead of digital it could also be out of calibration.. Like a speedometer in a car...does it really match?
You most likely have an out of calibration thermostat