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.