400watt hps bulb working in a 150 watt ballast

So i just realized i put the wrong bulb in one of my ballasts. It is a 400 watt hps bulb in a 150 watt ballast. It burns significantly brighter than a 150 watt bulb. Any reason this could be? Im no electrical engineer but this is strange to me. The ballast doesnt even have an ignitor. Just what looks like a transformer, a capacitor, and a few resistors.

If anyone has any idea id love to hear it? Am i using 400watts? Or 150? Would a 400 bulb light up ans stay lit if it wasnt getting 400 watts?

It has been running like this for over a month now, no overheating issues with ballast or funky operation. Id appreciate it if you skip the whole youre going to burn your house down bit. Only answer if you can intelligently explain what is happening.

Since the ballast is only 150W, the bulb is only receiving 150W. I'm not a super technical person, but I think you're safe using a bigger bulb. It's only when you try running a small bulb (150W bulb for 400W ballast) that I would worry about houses catching on fire.

Im afraid I cant explain off the top of my head why it burns brighter.

Maybe its just new bulb vs old bulb. I wounder if the lumens are the same 150watts through a 400 bulb and 150 through a 150 bulb. Good to know though could save me some money down the road.