There are many ways to skin this JPM cat but I will wager you that there is way more than meets they eye. If for one it's actually true that JPM lost $2B they probably came out with it to forestall future panic if what they lost is just the tip of the iceberg.
There's a different theory however. In March, JPM announced a $15B buy-back program with $12B earmarked for 2012. Further, they announced an increase in dividends. Now, both these actions lead to a reduction in shareholder equity on the books. Looking at today's numbers:
JPM share price dropped by 9.28% (-$3.78) from $40.74 to 36.96. Most of this happened pre-market.
Total volume for the day was 217MM shares (vs. a daily average of 30MM). If in their cunning ways, JPM bought all these shares at roughly $36, this would amount to $8.03B in total spend. Had the price not dropped from $40, the total spent would have been $8.8B. That is a $800MM saving, equal to 40% of the announced $2B loss.
Now, I am not suggesting a possible heist, but there is no mention of JPM having unwound those positions from their CDS Index "hedge bet" and there is no proof that JPM re-purchased every single share sold out there today.
That said, this is not beyond them and trust you me, there are some sharp knives in that drawer.