I noticed the attributes which modify the health aren't the same.
For example, in a "give" command, we use an attribute to modify total health to -90%.
For this, I use "Operation:2".
Command:
/give @p minecraft:diamond_sword 1 0 {AttributeModifiers:[

]}

Default world. No other commands. When I hold this sword, my hearts are reduced from 10 to 1, which makes sense. (10-90%=1)

Now to the problem part:
With loot tables we create the same diamond sword with -90% health attribute.
For this we create a new loot table. (DOWNLOAD the .zip and place into your world in /data)
However instead of "Operation:2", it uses the "multiply_total", and we add the amount -0.9.

Ingame, when we hold the sword the health is supposed to be 1 heart. (10-90%=1)
HOWEVER, it shows 2 hearts.

PROBLEM: It shows 2 hearts, bcs the code likes for some reason to change your amout you enter to a really close one. Example here: I entered "-0.9" and when the loot was generated, the item had an amout of "-0.899999..." (Look at the SCREENSHOT here.)

This was a test, basicly the generated items wont take the numbers entered 1:1, eventhough it is clearly defined in the loot table. The true, generated number is always a really close one to the entered number, which can mess things up.

Pool:

{
    "pools": [
        {
            "rolls": 1,
            "entries": [
                {
                    "type": "item",
                    "weight": 1,
                    "name": "minecraft:diamond_sword",
                    "functions": [
                        {
                            "function": "set_attributes",
                            "modifiers": [
                                {
                                    "attribute": "generic.maxHealth",
                                    "operation": "multiply_total",
                                    "amount": -0.9,
                                    "name": "Health",
                                    "slot": "mainhand"
                                }
                            ]
                        }
                    ]
                }
            ]
        }
    ]
}

I hope I explained it somehow clear enough to understand.