My theory is somewhat conjectural, so no need to take it too seriously. But it is at least based on a solid fact. I found that the Sony RAW file (ARW and SF2) Exif metadata includes a camera model code, stored as the Sony makernotes tag 0xb001. You can read this tag using applications like ExifTool or PhotoMe. The (decimal) codes are as follows:
2 = DSC-R1

256 = DSLR-A100
257 = DSLR-A900
258 = DSLR-A700
259 = DSLR-A200
260 = DSLR-A350
261 = DSLR-A300

263 = DSLR-A380
264 = DSLR-A330
265 = DSLR-A230



269 = DSLR-A850


[??? = DSLR-A500 Files not examined]
273 = DSLR-A550
The “holes” in the list suggest that six (or seven, depending on the unknown A500 code) additional cameras have at least reached the stage in which they get their own code. Some of them may of course never see the day of light. But others may be models with a long gestation period, like the A900, which was a announced a year after the A700 but has a lower code. They may be either DSLRs or other cameras that make raw files. Interesting, isn’t it?