First, it assumes that all counties will legalize commercial sales. Prop 19 doesn't make commercial pot sales legal, but simply allows the counties to decide for themselves. You'll be able to grow your own (tax free) in counties that don't allow it, but nothing will be commercially available there. A number of counties have already suggested that they won't be legalizing.
Second, the LAO assumes a $50 per ounce excise tax on pot, which was randomly pulled by a single proposal from one legislator and which most pundits are saying is unreasonably high. This is especially true in light of a RAND study released a few weeks ago predicting that pot prices will fall by 80% to 90% in California within two years of legalization. Current prices are largely driven by market forces (supply and demand), which will be upended once commercial scale marijuana farms begin operation in high production regions like the Central Valley. The commoditization of the crop will slash its value to as little as $35-$45 an ounce (currently, pot sells for $300 to $400 an ounce in California). If that happens (and MANY economists have chimed in since the release, saying the RAND numbers look about right), the LAO excise tax would amount to a 100+% tax on marijuana sales. That is untenable, and would simply lead to a gray market designed to avoid the tax.
RAND put the projected tax numbers at $800 million a year
if it's legalized in every county and
if a $50 per ounce excise tax is imposed, resulting in a $91 per ounce sale price, and IF those numbers lead to the projected 75% increase in the number of marijuana users in the U.S.
It's a pretty fascinating piece of work:
http://www.rand.org/pubs/occasional_papers/2010/RAND_OP315.pdf