initial preheat will usually be higher so it can help bring the oven cavity up to temp.
Door opening can drop the temp over 100 degress in under 5 second.
The cheap metal dial therm are GARBAGE.... toss them.
Most ovens use an average temp. Think Signwave, you select 350... it preheats first cycle to about 380-400. cools down to 340-350 turns back on to 360-380 cools down to 330-340 so on and so on till the average temp is 350. Hope I'm building a clear picture. MOST, (not all) ovens use this method of heating.
First pan of cookies takes about 12mins (give or take)
Last pan of cookies takes about 8mins (give or take)
Why you ask? because the cavity (metal body) of the oven is up to temp so the door openings don't effect the overall temp all that much. It can recover from those opening easier.