It seems like your back-of-the-envelope estimate is assuming that the market values UBS at exactly its assets minus its liabilities. This disregards goodwill, which (someone correct me if I'm wrong) includes expectations of future earnings. That number wouldn't even be affected in a straightforward way since banks, especially European banks, tend to be levered quite a bit in order to survive on interest income.
Goodwill is often counted as an asset (or a liability if they don’t want to pay taxes) on the company’s book. I’m not sure if UBS included it on theirs, though
