Why is Annie so tanky?

Seriously I'm really confused as to why as her stats and passive wouldn't suggest it, but just had a game where I thought "Ah Annie mid she'll be squishy" (play Hecarim so have the big charge in combo) but every time all that happened was the full combo did about 1/4 of her health then she'd unload almost all of my health in 1 combo. Now I get that she's meant to do a lot of damage, but the ult charge combo can 1 shot people going full damage, but was barely being hurt at all, is there some bug that's making her super tanky? Some Rune page setup people can use to become really tanky while still hurting people? or is she just 1 of those champions you think will be squishy, but in reality has really good base defensive stats that scale super well as she levels? I've learnt my lesson at least... don't try to kill Annie first as she'll just survive and smash you back much harder.
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