Someone on the board, I don't know who, floated the idea that the Constellation was an upgrade of an earlier class. Just as the Enterprise in TMP was refitted while retaining her registry number verbatim, the idea here is that so was the Constellation. But since the Constellation was evidently not the class ship, we would assume that the upgrade happened after the 17xx class was already established. This proposal might not necessarily be at odds with Jefferies's intended system, since there could be significant differences between the hypothetically refit NCC-1017 and new ships in the 17xx class.
Legibility, I imagine. The number NCC-1701 was chosen in the first place because it was an easy number to make out even with the vagaries of image degradation in FX shots, broadcast interference, tiny '60s TV screens, etc. Given those considerations, 1710 may have been deemed too similar-looking to 1701 to be easily distinguishable, whereas 1017 is much easier to recognize as a different number. Although, granted, the Constellation was pretty recognizable due to being so badly damaged.
^The lack of detail wouldn't have been that evident on screens of the day. Wobbliness would be expected given all its damage. And who's to say it's inaccurate? Different ships of the same class are often customized with different features.
I remember seeing the 1017 for the first time as a kid, and I thought it was the same number as the Enterprise. I even remember saying that out loud - "It's the same number!" My first clue, I guess, that I have a mild bit of dyslexia.
I pulled out my Star Trek sketchbook last night, and poured through the designing the Enterprise section. I found a sketch by Mr. Jefferies of a clearly near-complete Enterprise design that laid out the design/serial schema for 17-01 (and included A as an addendum for the first major refit.) The drawing isn't dated, but it's clearly by Mr. Jefferies and does indeed seem to come from early in the series. So, it does appear this was the initial intention; NCC meant Federation, the first number pair indicated which design the ship was, and the second pair indicated the ship's place in the series. Slightly further OT, but I just had the thought: to truly distinguish the Enterprise and Constellation, why didn't they use another design? They could have come up with some sort of kitbash from the AMT kit. The answer, I suppose, is rather obvious: production intended the Constellation to represent a ship of the same (apparent) class as the Enterprise. If they'd wanted her to be another class, she would have been. Having her be the same class probably provided the best sense of awe for the power of the Planet Killer, provided a real sense of danger to the Enterprise, and, from a production standpoint, was easiest. I suppose, then, with these two pieces of evidence, the only logical conclusion is that Mr. Jefferies wasn't the one to put the Constellation together and put the decals on, and may've been annoyed when he saw it. OTOH, Mr. Jefferies may have seen fit that Federation cruiser designs did not change in overall outward appearance much between Designs 10 and 17, at least to 20th century eyes.
^Not sure he would've been annoyed. As an art director in series television, he must've long since come to terms with the fact that he was not a sole auteur and that his suggestions were subject to revision by the people who employed him. Just because he conceived the idea that the "17" represented a specific class, that doesn't mean he expected all his colleagues to bow to his will. After all, he was an employee, not the boss.
Darn, you noticed! But I'm not yet ready to post my thread what I do think of using the nickname "Connie" for - the Enterprise? And after I've posted this thread, chances are high I become the ultimate persona non grata at the BBS and will probably be banned. Soon... Bob