Dear Daniel,
it is not true reflectances are between zero and one : physics allow to observe reflectances above 1
Of course the total irradiance reflected by a surface in all directions is lower that the incoming sun irradiance, which means that the integral of reflectance in all directions is lower than one. But the reflectance may be higher than one in a particular direction.
For instance, a window roof that reflects the sun right in the eye of Sentinel-2 can have a reflectance much higher than one.
Another phenomenon contributes to observing reflectances larger than one.
A snow covered pixel in a mountain facing the sun can also reach values above one. This effect is due to the fact that reflectance is normalized by the cosine of sun zenith angle (angle between sun direction and vertical). But in the case of a slope, it should be in fact normalized by the cosine of the incidence angle, which is the angle between sun direction and the normal direction to the slope. Sen2cor and MACCS have a “terrain correction” to correct for this effect.