Preheating oven increases temperature?
Sadly, yes.
Your oven most likely has been running too hot for a while and now that you are actually measuring it, you noticed it.
We have a lot of Q/As on the site that recommend using a separate oven thermometer whenever an oven behaves strangely. Thermostats can fail or be generally incorrect, like too hot or too cool. Plus many ovens fluctuate quite a bit.
If you noticed that your recipes didn’t turn out the way you expected or were used to, getting that oven thermometer was a smart move. Either adjust the temperature to the desired value, if possible (and note down what setting that corresponds to), or consider having the oven serviced or repaired. In some models, the temperature knob can be adjusted, which would be a super trivial thing and may even explain why the oven is set to a wrong temperature.
If your question was wondering more about the still rising temperature - ten minutes preheat is on the shorter end of preheat time, especially for old ovens. You can be dealing with quite a bit of thermal mass.