Without more information, I can only guess, but it sounds like an off-by-one error. In your OP: "65536 values from 0-254". That suggests a 255x255 square which is 65025 values; a 256x256 square is 65536 values. Do you perhaps want to start with "65536 values from 0-255"?
Update: s/off-by-error/off-by-one error/