Probably cause you're using other items that increases HP by percentage, like Tao Gunka Card. Valk Shoes increase base HP, and not by percentage. Maybe it stacks well with your other items that increases HP by percentage rather than having another items that increases HP by percentage.
Try removing all your items, and and test the difference.
My sinx (w/o any other items equipped):
Base HP: 155,340
HP with Valk Shoes on: 175,740 (difference of 20,400, that's 80*255)
HP with +10BG (although I only have +9, I just compute): 170,874 (difference of 15,340, 10% of max hp)
Some computations with HP (I don't have zeny to decard, so I'll just compute :< well, I just card my tao) using the same base HP: 155,340
HP with Valk shoes and Tao: (155,340 + 20,400) * 2 = 351,480
HP with +10BG and Tao: 155,340 * 2.1 = 326,214
In any case from the above, valk shoes gives more HP than +10BG.