Starfleet's style of uniform changes many times.
Unconsistant Inconsistent writing by the developers.
You must consider that between ENT and TOS, about 80 years passed, and then about another 100 years passed before TNG. It's a lot of time for styles to change back and forth. Look how many US Navy uniforms there have been over the past two centuries... using collar insignia, shoulder boards, and sleeve stripes to denote rank.