The short answer is that, unless you were making the reflected light reading from a standard gray card, you should expect the readings to be different. The reflected light reading takes the intensity of light within the field of view and, in effect, arrives at the setting required to produce that light intensity as "standard gray." If the object being read is black - you still get gray; if white- you still get gray.
The incident light reading measures the amount of light falling on the object and, in effect, calculates an exposure which would correlate to a reading made - in the reflected light mode - of a standard gray card.

The incident light reading, executed properly, will normally tend to render the scene fairly accurately (assuming normal development) while the reflected light reading will depend on the angle of view of the meter (spot or wider angle), tonality and reflectivity of the object surface measured.