It is the fast lane in California. According to the California Driver Handbook, the left lane is known as the "fast" lane (p32). They suggest if there are three lanes to use the left lane to drive faster, pass, or turn left. They also say "Do not drive slowly in the left (fast) lane." (p64)
The California fast-vs-passing lane pattern also emerges due to two
things that California requires that other US states tend not to require:
semi-trucks must go slower and keep right.
From the handbook cited above, or see also signage on many CA highways:
The maximum speed limit on most California highways is 65 mph. You may
drive 70 mph where posted. Unless otherwise posted, the maximum
speed limit is 55 mph on two-lane undivided highways and for vehicles
towing trailers.
...
When you tow a vehicle or trailer, or drive a bus or three or more axle
truck, you must drive in the right hand lane or in a lane specially
marked for slower vehicles. [emphases added]
On divided highways of 2 or 3 lanes in each direction, such as I-5 in
the Central Valley, these requirements strongly separate traffic into 2 sets -
semi-trucks doing 55-ish in the right lane, and non-trucks doing 65-70-ish
in the "fast" lane. (okay, yes, 85-ish, except in Kern County).
The results are that 1) over-the-road truckers dislike driving in CA,
and 2) CA non-truck drivers quickly learn to avoid the right lane.
The other 49 US states tend to have more of a "keep right except to
pass" pattern, depending of course on other local variations,
especially population density, traffic volume, and similar.
Source: someone who drives (and tows) in 18+ states per year.
http://www.dmv.ca.gov/pubs/dl600.pdf