The unused bits are always read as zero so no matter which justification or 'shift and OR/Add' should leave a bit stuck at '1'.
It certainly looks like a configuration error, most likely due to the clock being set several times faster than recommended. I would guess (only Microchip know exactly how it works) that reading back the ADC before acquisition is completed leaves a residual bit set in the comparator tree and that's what gets returned in the result.

Brian.