Probably various factors. Chambers clearly had something to prove indoors, hence the 6.42, then felt like he had achieved it and took his foot of the gas a bit. Pickering was slightly injured outdoors in 2007 (off his 6.55 indoors) and so couldnt do any block work from June onwards. Yepishin ran his 6.52 at a home championships (the world indoors) which I always find suspect!