Because whatever it is, that's what I'm writing to my oven. I hate my oven. For a while we didn't think it was pre-heating to the correct temperature so we bought a thermometer and sure enough, it wasn't getting hot enough. I ended up having to set it to about 15-35 degrees hotter so it would get to the right temperature. It worked fine that way for a while, but now it's gone crazy. A few weeks ago I set it to 380 so that it would heat to 350, like normal, but instead it heated to 385. Two weeks ago I set it to 425 and it beeped at 300. No matter what temperature I set it to it wouldn't get above 400. The next day it was fine and heated normally. Yesterday I set it to 350 and it beeped at 295. I turned off the oven, set it again and it worked fine, heated to 350.
I can't wait until we get our own place and can buy our own appliances.