That is literally what it is. It was the center point of the peoples (anglo, germanic, frankish, italian mostly) on whom the testing was done, and so as with the metric distances being pegged to (attempted) measurements of the planet, IQ was pegged to our centerpoint. If they wanted an order of magnitude more granularity without using decimals, the centerpoint would be 1000 -- but it would still be based on whites.
You seem to think that it's a specific score that was achieved by a specific group of people at a certain point in time, but this is not true.