When you say oven thermometers, what type? Was it an oven dial thermometer, or a oven cord thermometers, or a remote oven cord thermometers. The dial thermometer you need to open the door to read it, thus you will loose heat. Also the dial types are not accurate, and can take up to 2 minutes to measure the temperature. Also Cook's Illustrated has an article on dial oven thermometers, and in that article they state the industry standard allow the readings to be off as much as 25°F.
The last two mentions, the probes are attached to cord that you run through the closed door, and the other end is attached to a base unit with a digital screen attached.
So it would be helpful if you mention the type of thermometer you used.