Because +HP affects HOW MANY HP (or fractional HP) you regenerate in a set time.  
	Tank.  Bare build, no enhancements. 
	HP: 1874 
	With auto-Health, that's 140% base regen.  10.93HP/sec  
	Now, just turn on DP. 
	HP: 2523 
	With auto-health, that's STILL 140% regen. 
	BUT, because the base HP is higher, 140% on 2523 = 15.3 HP/sec  
	Yes, more HP regenerating, but your actual REGEN PERCENTAGE VALUE doesn't change.