For one thing, because Tormolen told Sulu "You don't rank me," meaning "You don't outrank me," so he and Sulu were evidently meant to be the same rank. So it's inconsistent with the dialogue. For another, because it's the only use of that kind of rank stripe in the entirety of TOS, and no TOS-era character was ever identified onscreen as a lieutenant junior grade. Something that's used only once and never again is often just a mistake, or an idea that was tried and then abandoned. Proof requires repeatability.
True, the first season has a number of examples of the dialogue rank being different from the wardrobe rank, and this could be another example. But the system was made sense of eventually and the dialogue made to fit. Tormolen's insignia existed and fits the scheme, it had to have meant something or the effort would not have been taken.
Going through my old reference sources, I find that both the Concordance and Shane Johnson's Uniform Recognition Manual assert the existence of a broken-stripe lieutenant j.g. rank braid, but the Star Fleet Technical Manual jumps from ensign to full lieutenant (which is how I always believed it to go) and aridas sofia's Federation Reference Series oddly assigned the half-braid to ensign and the absence of braid (like Chekov and other onscreen ensigns had) to "ensign appointee," whatever that means.
The FJ TM is obviously not reliable as it shows three "regular" stripes for commodore, rather than the broad band seen many times (though with differing details) in TOS.
"Inconsistency" doesn't mean "accident." It means that incompatible decisions were made in different instances, whether because they were made by different people making different assumptions, or because the same people changed their minds or forgot what they'd done before.
Yes, for instance Theiss may have devised an insignia for LTJG but it never came up in the writing so was never used again.
You're talking about Starfleet as a conjecturally "real" entity. I'm talking about Star Trek as a television show and the intentions of its creators at the time. The point is, there's no clear proof that the creators of TOS intended a lieutenant j.g. rank to exist in Starfleet. Which is why fan references from the '70s and '80s disagreed on the question.
No, I'm saying the US Navy was a real known entity and it would make sense if Theiss devised a system that used that template and included that grade, just as TNG did, and the OS system of stripes would perfectly fit six officer grades (below flag) starting with zero and going up half steps to 2.5. Indeed, it would be odd to choose 2.5 for captain, rather than 3, if it were otherwise.
If LTJG did not exist at the time, one also has to imagine a fictional reason that the grade was restored, or removed and restored, which doesn't make much sense. Anyway, I can see why it may have been an inconsistency, but I'm not seeing how it was "probably" an inconsistency.
One interesting tidbit: in the TMP braid system, 0.5 (dashed braid) stood for Ensign, but everything above that at least up to Captain was the same, and there was no LTJG. Two NCO ranks were specified below Ensign. I like that better than the concept of enlisted personnel. IIRC, somewhere in TMoST there's a line about all astronauts being officers.
Enlisted personnel were always there in TOS, with numerous onscreen references despite GR's TMoST comment. As
Longinus said, the idea that ensign and everyone below that wore the same insignia (that is, none) is a little hard to fathom. TMP's scheme seemed to be an attempt to remedy not having an insignia for ensign, but consequently disregarding JG, even though that grade was used in the novel.
TMP's enlisted scheme is more like the British Royal Navy, with only the two grades of petty officer, CPO and PO. For TWOK, they went with a proliferation of enlisted grades which is another jarring shift from one movie to the next.
