[Yo-Yo]

Any idea why the F-35B is farther ahead than the A and C variants to the point that the B will achieve IOC prior to either?

Lockheed Martin focusing on the B model since it was the most critical version coupled with the Marine's definition of IOC being much lower than USAF or USN's definition of IOC.

[A.A. Cunningham]

The services are driving the focus not Lockheed and sequester is dictating what gets priority and what doesn't. NAVAIR is for all intents and purposes only mildly interested in the C and the same can be said for the zoomies and the A. During the Bs "probation", testing on the A and C wasn't accelerated. The zoomies have adopted a different philosophy and learned from the mistakes they made with the F-22 and NAVAIR would be quite content with buying more Super Hornets.

the Marine's definition of IOC being much lower than USAF or USN's definition of IOC.(sic)

Much lower? That's highly subjective and history shows us that the Corps has always been willing to get into the fight with a Chevy instead of waiting for a Cadillac, unlike her sister services. Even the zoomies have shifted their IOC plateau opting not to wait for 3F; they'll go with 2B or 3i, unlike the Navy which is quite content with waiting for 2019. If they select 2B they'll be changing their IOC definition to the "much lower"(sic) definition used by the Marines.