First - I do not put any trust in those really cheap thermometers with the dials, they work off a coil of wire that expands and contracts according to the temperature - and they are the least reliable, and there is no way you can easily check them to insure they are within calibration limits. For oven use, I would highly recommend a wet-bulb type thermometer, such as this one from Taylor called the Oven Guide:
They don't get out of calibration and only cost about $15. If you can't find them locally you can get them at Amazon.com or other online sources.
Second - as wysiwyg brought up ... ovens do have different temperature zones ... if you move the thermometer around to various areas (front/back - left/right/center) you will see that - how much difference you find depends on the oven.
If you can locate where the thermostat probe is located within the oven, and place the thermometer next to it, then you can guage how close the oven's thermostat is reading the temperature.
I'm with Uncle Bob - if everything has baked fine and your "Thrift Store" thermometer shows your 20% off ... I would really question the thermometer.