Maybe the technology is clever enough to only change the bits that are different, rather than updating it all.
So if the car goes along the road photographing everything, and most of it is the same, nothing changes. But if something is "new" it gets updated.
Funnily enough I noticed this just a few days ago.
There is a major road where I live in Birmingham where a load of new buildings are being built. When you are at one end on Google maps street view none of the buildings are there, but as you move along the road the buildings all of a sudden appear.
As you rotate round the buildings come and go, it is really strange.
Here is a link (which I hope works). Just step alond the road following the yellow line, and new buildings appear on the left and right (you may need to rotate a bit to see them)
http://tinyurl.com/dy99uhh