It's a clear sign of one of two factors:
- Age – Older outlets in California are two-prong, often ungrounded. They're still exceeding common, given the cost of electrical retrofits, and the secondary issues they may reveal during inspections (plumbing, seismic) that would also need fixing.
- Low-amp Circuits – Sometimes, people try to do really crazy things with standard apartment electrical outlets. To help prevent this, apartment management outfits most of the outlets (except the kitchen and bathroom) with two prong outlets, to make sure higher-amp electrical devices (grow-lights, hot plates, table saws) are not used within their apartments. Even if we're not talking about burning the place down accidentally, reduce the power output capacity, and you help reduce the potential noise factor for neighbors, which is less headaches for management. The basic idea is, if they give you higher amp protected circuits, you'd automatically abuse them. It's simply not worth the perceived risk for shorter term (sub 2-year) leases.
Depending on the nature of your apartment, it's much more likely to be the former – though some of the short term lease apartment complexes/chains do have a sprinkling of the latter as well.