According to Scott Schindler and John Eaves, the difference is do to legal reasons, they couldn't use the TOS design. It had to be at least 25% different because of copyright.
https://www.facebook.com/john.eaves.526/posts/2039419259419297 it must be all in context, because they can still show the Enterprise on book covers, in video games, comics and toys.
Also the design we got in the finale episode wasn't Eaves' original final design, the VFX department made their own changes.
Nor is the calendar one above final, they fixed that up in Photpshop to make it match the aired version. That explains the ghostly second deflector antennae.
Here are the differences between his version and the show version according to him:
The changes between the two enterprises are as follows. The new ship has more TMP struts than TOS struts, the main hull and nacelles are shorter and more plump, the deflector dish now has one antenna vs two, the impulse module is thinner from side to side, the overall ship has a heavier plating detail, and the exterior has a more broader range in lighting and nacelle glow. I am sure there are subtle differences but I have not seen any more than you have to make a more detailed comparison so look at the calendar as concept art and all will be good and hope this quick breakdown helps.
Here are couple comments on the design:
Scott Schneider - "...We had straight pylons. We never considered the swept because we felt it would be jumping forward in time for one element. Having said that there are elements of the refit design that really should have been in the original such as an airlock/docking port. I also added hatches to the bottom of the saucer that slide open to reveal additional airlocks. These werent added so much as copying the refit but rather that they just make sense as a functioning ship. I like to approach things from the standpoint of practical application"
Also John Eaves gave explanation for the split in the struts:
John Eaves - "...we split the struts so in time the cooling vent side could be removed to make it more like the Original TOS strut."
And to round that off, Jonathan Frakes spoiled plot details again, this time for Season 2 Episode 2, so:
SPOILER WARNING
https://i.imgur.com/QRdV6AN.png