Silly question, but... if the Kelvin is contemporary with the 1701 in the film, why would the crews wear anything other than the same uniforms as the 1701 crew, but with different insignia?
It's not contemporary; more than one time period is depicted in the movie. The ship itself is supposed to be from a design generation before
that of NCC-1701, and the events involving the Kelvin
are supposed to include James Kirk's father among the crew and take place at a time before our Kirk's birth.