A reflective and incident read differently, so a 1 1/3 stop difference does not necessarily mean anything is wrong. But a 1 1/3 stop difference between one meter and another like meter is a red flag that something is wrong.
Originally Posted by stradibarrius
Both can be very accurate but under different circumstances. The incident meter is accurate for the reflective surfaces in the subject if those surfaces are receiving pretty much the same intensity of light----such as even sunlight on snow or subjects in full shade. But using the incident meter in sun/shade mix situations will overexpose the high values if read in the shade and under expose the shadow values if read in the sunlight.
An incident meter is also quite useful in the studio when measuring the light intensity of individual lights, in this way, the power of one light can be set to provide a stop or two more or less powerful than any other light.
A 30 degree angle reflective meter, like my Gossen Luna Pro F (or in-camera center-weighted meter) can be accurate for the reflective surfaces in the subject if there is about equal amounts of light versus dark areas being reflected. But, if there is more dark than light or light than dark, then these readings can lead to poor exposure too.