Well there are a few variations of the TOS Enterprise-type ships.
1. The TOS Enterprise herself, NCC-1701 as seen in Seasons 1-3.
2. A line drawing of the Enterprise as seen on the displays in "The Enterprise Incident" that had a beveled saucer edge and warp nacelles in a different position. I like to call this the "Pre-(Captain) Pike" version.
3. The USS Constellation, NCC-1017 which was based on the AMT? kit which had slightly different shapes for the hull.
It could very well be that the Constellation is a different class, or the same class but not upgraded, or a variant of the base class, whatever that is.