The original PocketBook readers only supported 4 greyscales in hardware, but (like most EInk devices) they used dithering to increase the effective greyscale range. PocketBook claims that the 302 has 16 greyscales - but then they previously claimed that devices with 4 hardware greyscales also had 16 greyscales.
If an image only contains 4 grey levels, then that is all you will get. See
More display issues on the DR for some images and ebooks to use for testing.