That doesn't make sense. If Nvr supplies up to 15.4w then why would it supply more. And if cam requires more power then 15.4, wouldn't the camera just not function? I have 3 of these connected to my Nvr and is fine. I doubt 18w is correct but I will test it to make sure.
It makes complete sense. It would either kill the PSU or make the PSU voltage drop so much that it would be sufficient to power the NVR anymore. This is basic electronics.
The camera you have is max 18W, to use that it will have to have all of the IR Leds on max and be using the zoom and pan/tilt etc. So what is happening is you are using less than 15.4W and that is why it hasn't gone boom - yet. On top of that there is a maximum combined draw over all PoE ports. There's more chance the PSU blowing than the camera stopping to work. The camera will just try to draw what it needs. If it need 18W and the PSU absolute maximum is 15W, then it will try to draw 15W. If it shuts-down then great, if not, boom your PSU is dead. It might not blow immediately but as you are running it outside the specification, it will get hotter than it wants to run and will one day, just give up the ghost. I know this from first hand experiences of PC power supplies, which are very similar to the NVR ones.
I have a DS-2CD2032-I connected up at the moment and it says 5W in the dark, despite saying 7W max. I'm sure if it turned up the IR it would go to 7W.