There can be a huge difference between what a bank says its assets are worth and what they will fetch in the market.

The latest reminder: a deal announced on Tuesday by Morgan Stanley and Citigroup that valued their brokerage joint venture at $13.5bn. That is almost $10bn less than what Citi figured the Smith Barney venture was worth this spring and will lead the bank to take a non-cash, after-tax charge of $2.9bn in the third quarter.