I only try to find reasons on why this may not be a bug, to eliminate any confusions, mistake, and make sure it's really a bug.
######I'm assuming you're referring to a game you had with Blitzcrank (and Vayne/Eve/Annie).
There is only one thing I'm able to find how Sion's scores can give one a wrong idea about the bonus health =(.
(Blitzcrank)'s item, or at least any items that contains the component of
, can be used to give an ally coins, including a score count your CS (creep score).
This score count will not contribute to your bonus health, as it probably does not count as YOUR kill or something (I'm not going to argue about any logic over this).
At 6 kills and 14 assists, you SHOULD receive 200 bonus health from this according to his ability's description. You typed that you were only getting 540 health. So... 540-200 health equals** 340 **health.
You said you had around 200 CS at a certain point. Not considering large minions, it seems like you should get 600 bonus health. That's 260 excess health missing.
In order to make up for this with the Targon's set of items that Blitzcrank has, he'd have to kill 86 minions (remember we estimated the other stuff, so it is not a true/exact situation and number).
So I have to ask, by any chance
(Blitzcrank) gave a significant amount of minions that may have you thought this as a bug?
The above scenario does not take into consideration that you may have made an estimation, mistake, lie, or imagination, about the things you have typed. In this game, I think it really would help if you have a recording of the whole game, or something. I think it would be too difficult to miss the cause of the situation you're experiencing.