This applies for Rockchip RK2808 and RK2808 devices dating all the way back since April 2010.
RK28xx devices typically come adb shell rooted likely due to negligence.
All known oneclick rooting methods such as SuperOneClick, Universalandroot and Z4Root exploits do work to some extent, but the system is mounted cramfs, which is a compressed read-only file system. Typical Android root root relies on the ability to write files to modify the behavior of the system. In otherwords, the su binary and the Superuser handling application cannot be installed to the system.
To permanently root the device, you need to manually repack the firmware: [How To] Unpack and Repack for Custom Firmwares for Rockchip RK28xx, and then repack the firmware.