Training
Imagine, just imagine that Dave B were performance limited. i.e. the engine geeks were getting about as much from the donk as the airframe geeks had made their structure weigh. That would mean that every deck landing would be a one off, and the pilot would then taxy to the chocks on vapour. So to do five or so would require the same amount of turn rounds etc on deck (i.e. deployed/ detached)
Imagine, just imagine, if Dave C were able to pitch up to the boat with enough gas for four touch-and-goes followed by the real deal/ RTB. It could even do it unsupported, and be back at Lossie before the rubber marks cooled.
If the above were true Dave B would be ridiculously hard to actually derive training benefit from, whereas Dave C could fit a carrier qual or two into just about any mission you care to mention.
Of course, the VSTOL performance of JSF B has never been in doubt, and anyway, aeroplanes tend to get lighter as kit tends to be taken away during the platform's development....err, no wait.