why don't my perk bonuses add up? max health 1.39+1.39=1.62?

None of my perk bonuses add up to what I expect.

For example, the max health perk gives me 1.39 the first time I take it. When I take it again, the resulting value is 1.62. I guess I assumed that taking max health twice would give me 1.78. Third take gives me 1.86.

What's happening?