Lot of stuff written about Sony raw conversion recently. Here are some articles actually describing it:
A similar compression scheme is used on Nikons, here is Emil Martinec opinion about it:
NEF "lossy" compression is clever - Open Photography Forums
This article explains it a bit more:
How Many Bits to Fully Encode My Image | Strolls with my Dog
Jim Kasson has built a model of Sony's raw compression.
This is image the difference of the uncompressed image and the compressed image with no delta coding and dithered by shot noise at 100 ISO. The difference image is pushed 10 stops.
Now, if there was no photon noise present, there would be artefacts, the dithering effect of photon noise makes this compression scheme workable.
But, Sony has also a delta encoding scheme, handling a 16 pixel block of data and using a 7 bit coding of the range of numbers within that block. So, minimum value is saves as 11 bit code and the offset is coded by 7 bits. This compression is lossless unless the gradient is to large. This can cause artefacts:
The images above come from a series by Jim Kasson, here is a link to one of the latest articles in the series:
Removing the delta modulation component from Sony raw compression | The Last Word
Finally, here is an article on RawDigger discussing the issue:
RawDigger: detecting posterization in SONY cRAW/ARW2 files | RawDigger
This is the "star trail image"
And this is the effect of the delta coding, heavily enhanced:
Note the similarity to Jim Kasson's image, with delta compression enabled and photon noise on.