The a16z website shows the current portfolio (most recent, un-exited investment) and does not display their entire investment history. If you check out their Crunchbase profile[1], they have already had some big exits with Groupon, Zynga, Skype and Instagram, which is quite impressive since they only started investing 3 years ago. I'm sure there have been some other medium/large M&A liquidity events as well but it usually takes 3-5 years to figure out if your VC portfolio is any good and 7+ years to realize any returns. LPs know this and funds usually have a 10 year life cycle, sometimes with options to extend it a year or two [2].
[1]http://www.crunchbase.com/organization/andreessen-horowitz/i... [2]http://www.amazon.com/Venture-Deals-Smarter-Lawyer-Capitalis...