Percent HP boosting cards are an increase from your base HP without any Percent boosting cards.
If your HP is 8000 then your HP with:
DL; 12000 ( gain 4000 from DL)
DL, DI: 12800 ( gain 4000 from DL, 800 from DI)
DL, sleipnir: 13600 (gain 4000 from DL, 1600 from sleipnir)
DL, sleipnir, DI: 14400 ( gain 4000 from DL, 1600 from sleipnir, 800 from DI)
Regardless of how many percent HP boosting cards you use, each individual one will always give you the same amount of HP as long as you don't change your base HP with something like vitality adding gear.
The reason why it gives you less then 10% more HP when you equip it from your point of view is because you are seeing the percent more from the already increased number, not the base number.