Say, the Mercury astronauts were all officers from USAF, USN or USMC. Of the Navy guys, Shepard and Schirra were LtCmdrs but Carpenter was a Lieutenant; all the USAF pilots were Captains (the same as LtCmdr); Glenn, the only Marine, was a LtCol (the same as Commander). Since they were all hired to do the same job, we might speculate that USAF doesn't have any Lieutenants since their guys don't show the variety the USN guys do. Or we might decide the USMC skips Captain (that is, LtCmdr) rank because why else would Glenn be justified in being higher-ranked than the rest of the bunch? But a selection of seven heroes isn't statistically enough to make such conclusions - and seven heroes is what we generally get in Star Trek shows, too...
Minor nitpick, the bolded part is incorrect. Army/Marine/Air Force Captains are the same rank as Navy Lieutenants, not Lieutenant Commanders.
EDIT: Ah, someone beat me to it!