Lets make an example. You are wearing a helmet that gives you +45% Speed. If you find Chestplate with +35% and you wear it, you will lose speed (you will have +35% Speed) Instead of having +80% of Speed (45+35).

Not only happens with combinations of armor, it also happens with armor + item in hand. So if you have a Feather with +20% Speed in your hand, and find +5% Speed Boots and wear them, you will have +5 Speed instead of +25%.

This bug happens with Speed attribute and I think with MaxHealth happens too.