Well, they were actually following a sequential series until recently.If there's an inconsistency, I chalk it up to either politics or opsec. Just like real life. You ever try to make sense of US aircraft designations (i.e. F-15, F-16, F-117?, F-22, F..... -35?!?) ? Yeah, same thing.
The Phantom II in USAF service started life as the F-110 right at the point the government decided to start over, but instead of starting at F-1, they went with F-4 because that was the Navy's original designation for the Phantom. Somehow the F-111 slipped thru, but from The F-5 on it was in sequence for both AF and USN fighters. Gaps can be attributed to canceled projects.
I think the F-117 may have been so-named to created confusion, but I THINK it fit the old legacy sequence as well*. Things went smoothly thru the F-14, 15, 16, 17 (canceled and developed into the 18), 20, 22 and (YF)23.
Then for some crazy reason the JSF program gave the two prototypes X numbers, the next in line being X-34 and X-35. When the X-35 won the flyoff, some schmuck decided it would be "too confusing" to name it F-24, so they carried over the X-35 designation and made it F-35, thus screwing everything up.

Same happened with the bombers. We started the numbering over at B-1, then came the B-2, and now some yutz decided the "21st century" bomber should be called the B-21.

*EDIT: Okay I read Kamen's post after I wrote that. )