I can sort of understand why these horses would show a decline compared to their Derby figs. If you take pace into account the Derby pace was fast but nowhere near as fast as it appeared because Churchill was lightening fast that day. The Preakness pace was insane given the super slow condition of the Pimlico track for the Preakness. N layed off a fast Derby pace but battled through the crazy Preakness pace. Ex also was closer to to the much faster Preakness pace then faced in the Derby, though not being as close early his figure declined less. Stradi pretty much mirrored Ex's early pace which was much faster than the powder-puff pace he set in his previous win.
Having said that I still think that the figs showed too large a decline which I believe was due to underestimating the variant for the Preakness where the track had turned into a bog.