Why don't any of my electric ovens actually keep the temperature that is displayed?
A variance of 50 degrees or more - both high &amp; low - sounds extreme.
However, a consistent offset of 25 to 50 degrees is actually not that unusual. Hypothetically, that could be 35 degrees miscalibration of the thermostat combined with a 15 degree temperature swing between cycles. I don't know what the typical temperature swing is for a residential oven. For a typical HVAC thermostat, a typical temperature swing is +2, -1, or 3 degrees total - so 5 degrees if you include the extra degree in each direction to actually trigger any action. I am sure an oven will have a wider range - the chicken or cake is not as sensitive to minor variations as people are.
Thermostats are manufactured to behave a particular way, but I highly doubt they are factory calibrated, just as mass-produced cars aren't taken out on the highway before being delivered in order to make sure the speedometer accurately reads 65 MPH.
Many ovens now have a user-accessible calibration feature. For example, Kitchenaid has a page describing the calibration process.
You can also determine your oven temperature with sugar! You don't actually need a thermometer. (And thermometers themselves are often miscalibrated.) A quick search finds How to Test Your Oven Temperature with Sugar, though I first read about it in The Science of Cooking:

which is well worth reading for anyone interested in both science and cooking.