Nice puzzle. The first two comments below caught my eye. Based on the first I assumed you are quite tall, based on the 2nd I started questioning that again. Your reference to powerlifting made me wonder as well. So the advanced search function from this forum could be used to pull up a 3rd quote from you from a different topic.
The short answer: 250-260W FTP for a 180cm male is smack in the middle of the Bell Curve (or slightly exceeding) for the volume you are training. What is not average is the volume of your muscles (and crank length).