Thanks for the comments. I really like aalenfae
's idea that it is named after a previous ship. Yet the issue with that theory is that there is no "A" after the number. My original theory is that registry numbers are not necessarily based on the order of production. Rather they are based on the overall purpose of the ship. This might explain why, in the 100 years since Starfleet was founded, the primary registry number is only 17. So according to my theory there would be 17 primary types of ships used in Starfleet. Lets say for example that the 10's were a cargo type ship. The interior configuration of the Constellation may have been gear towards cargo transport(This is just an example and not necessarily what I think it is). This would explain why it was 1017 rather than 17--. But at this point we could arbitrarily assign any primary registry number to any general ship class.
As far as the look of the Constellation. After reading the thread about why there isn't alien technology in Starfleet designs; I came up with a theory. Suppose each planet has their own ship production facilities. Starfleet could send the ship specifications to each production facility and then each planet builds the ships. With differing construction techniques each planet might produce slightly different ships. So they would all be the same class yet look a bit different. Perhaps the the Constellation was built at a Vulcan or Andorian construction yard.