Because normal loot table is there. Hard mode has two loot tables: normal mode loot table and hard mode loot table. You get reward from each. If you did normal already, then there is no rewards from the normal loot table because you already got it. If you haven't done it, then you can get a reward from there (you can still get nothing because "nothing" is one of the "rewards" from the loot table). That's why you got those boots. They were from the normal loot table, but that doesn't affect what you can get from the hard mode loot table.