Speculation from someone close to him of why Brad was maybe finishing in the grupetto comes in a quote from Cav's book, in the Epilogue, actually relating the aftermath of their failure in the Madison at Beijing.
Reading (and obviously believing) this quote, from someone who knows Brad probably as well as anybody other than his family, his GT progression makes a lot more sense. It wouldn't have been possible for Brad to race hard enough on the road, as he would have had to in 2002-2008 against the number of doping riders we already know about, to get good results, without 'learning to suffer' or having his 'back against a wall' in a number of races, if not all of them. It does therefore look very much like, at least in Cav's opinion, he didn't bother to try.
I know for a mainstream cycling fan this seems nonsensical, the idea of not trying as hard as you can in races on the road, with the road being so much more important than the track, but from Brad's point of view it must have made sense. Concentrate on prologues, make the breakaways for your team if you can and if not wait for the grupetto to form. You put the show on for the sponsors, which pays your wages and concentrate your effort on the track where it's comparitively easy for you to do well.
The half an hour thing is fairly meaningless in itself, as grupettos seem to usually come in just before the cutoff time, i.e. around half an hour on a mountain stage. Cav and Bernie came in about 33 minutes down on la Toussuire stage in the TdF.
It looks to me like, after his almost accidental climbing breakthrough due to his track weight loss in 2009, and some on the job race training by CVV about what to actually do now he wasn't getting dropped anymore, (he wasn't even supposed to be Garmin team leader in the Tour, and given his performance in 2010 quite possibly wouldn't have coped well if he had been from the beginning) resulted in the fourth place, that the disastrous 2010 Tour performed the dual role of teaching him to suffer physically but keep going, keep fighting when he wasn't on form, and provided sufficient mental suffering to make him determined not to coast anymore, the coaching focus changed to specifically address his racing weaknesses, and the 2011 Dauphine etc. was the result.
Judging from some reports out of Mallorca he's maybe taking this a mite too far now into borderline masochism and I hope the team has employed someone to sit on him to stop him overtraining this year as I want to see him win more races