Perhaps most people "agree" on the amount of 800K bitcoins, because its what they've been told to believe. The only real sources for this information are a leaked database and the bankruptcy filing which blames the problem on a "bug in the bitcoin system." This latter claim has more or less been proven false.
It's far easier for me to believe the smaller amount of bitcoins. 200K BTC are still worth in the neighborhood of $100 million. Any much larger sum of coins would seem ridiculous for a three year old company such as Mt. Gox.
You have a point there: the number "800 k" ultimately comes only from a highy suspect source, that has flatly lied to customers in the past.
But, if the actual sum of all balances was only 220'000, it means that the balances of some clients have been artificially inflated, by 600'000 BTC in total. A malicious management would have an obvious motivation for doing that: namely, ensuring that those special clients will get awarded the lion's share of the 220'000 actual coins, at the expense of other clients.
I hope that Mr. Kobayashi is aware of that possibility, and is aware that many clients are aware of it.