I might be missing something here, but if you configured ADC for 10 bits, you will most likely want to only read those relevant 10 bits from the register. Are you doing this? If you are using the C programming language, this is done by the bitwise & operator. If you are using assembler, it is done by the AND instructions.
It is not possible to reduce the 10 bit result to 8 bit without losing accuracy.
Could you post the code?