I would imagine if you take dex in the morning, each hour thereafter, your 17 OHP levels are different.  They would start off as non-exisistent as the levels of dex in your blood are very high.  It would stay that way for quite some time.  Then at some point a few hours later the levels would fall low enough for the pituitary to sense that there is a problem---or maybe there is not enough cortisol.  That is where your body makes up for what it did not have in the first part of the day.  This could be way into the day because like I said it can take a hour or so to peak, and of course it does not fall so rapidly in the blood as HC or Pred does.  Perhaps it is for arguments sake only at say 2pm that the pituitary senses there is a considerable drop in cortisol and at that point only is where you would start to make 17 OHP.  Now even that would not climb rapidly I don’t think.  Within two hours it may very well be around 71 ng/dl.  However, you have to realise that thereafter, the momentum picks up and with each hour you make very much more--espeically in your active part of the day.  This may slow down as you wind down at night, but by morning--and esecially after the 3am ramp up---you will have far more elevated levels than what were read in the labs  at 4.30pm the previous day.  (That is taking into account that you do the next bloods at 8am without your dose beforehand). 

It is the best I can explain to you.  It is like peeling an onion.  At first when you peel off a few layers, you don’t see it getting that much smaller.  But as you peel off after a while even just peeling off one layer can make it appear a good amount tinier.  Well I am taking that example froma URL above but the same principle could be applied for hormones taken orally that deplete as the day goes on.