It can be taken 2 ways. Only for sorcerer levels or this way which I would argue it gets.
Say you take yor first 5 levels as sorcerer. 1st level 2 extra HP, 2nd 4 HP, 3rd 6HP, 4th 8 HP, 5th 10 HP. Now on your 6th level say you take something else, rogue for instance (hypothetical). At that level you would only get 10 extra HP because it is 2x your sorcerer level which is 5. At level 7 say you take another sorcerer level, you would then get 12 extra Hp because you went back and added another sorcerer level.
That is how i read it though.