As you have no doubt noticed, we run healing (i.e. Hit Dice) slightly differently in RothP. 

We all took part in the D&D Next play test which we enjoyed greatly.  It was very rewarding to help shape the final form the game would take.  The one thing that we disliked about the final form the game took however, was healing through Hit Dice.  Actually Hit Dice we liked, we liked them a lot!.  The ability to heal up outside the use of magic was great.  Back in first edition, you gained one HP back for each day of rest.  It was a realistic mechanic, but if you didn't have a healer in the party it was a long down time to get back up to full HP.  Hit Dice was a great way to speed up the healing time, extend adventuring time, and to simulate manual first aid.  After all there is plenty you can do outside of magical healing to mend yourself even in our mundane world! 

There were however, two things we didn't like about the final form Hit Dice took.  The first was gaining all of your hit points back the next morning.  It made no sense if you were burnt by dragon breath, splashed by acid, raked by claws and teeth, that you would wake up the next morning fresh as a daisy.  The second was using multiple Hit Dice at once.  You were at 1 HP and then applying some bandages and a tincture zapped you back to full.  It didn't seem realistic.  So, we came up with an alternate plan.

In RothP, you instead wake up with all of your Hit Dice each day, but don't regain any hit points overnight (but you can spend HD right away).  You also can only expend one hit die at a time during a short or long rest from levels 1-5.  At levels 6-10 you can use 2 at a time, 3 at levels 11-15, and 4 at a time at level 16+ (this gives you a chance to do first aid 4 or 5 times a day which is realistic).  This is to mimic applying mundane first aid (applying/changing bandages and ointments, rubbing out the bruises, etc.)  We felt that this arrangement gave us a more realistic healing profile.  Barring magical healing, you slowly got better over time as we do naturally aided by the manual means we have to speed up the mending process. 

From the Dungeon Master's side of the screen this is also desirable as it means your characters won't do as they are wont to do - going into the first few combats of the day fully healed and then retire to somewhere safe as all of their Hit Dice are expended.  It also heightens the tension (and makes it more realistic) as characters tend to remain slightly damaged.  Characters that take damage in one combat are likely to be down a few hit points in the next combat.  Knowing that their cushion is that much smaller makes for a grittier game play experience.