B.E.T. Sorption mode.
It may be the case of some light condensable gases like carbon dioxide at high pressures in olefinic polymers, The gas has a light affinity for the polymer, maybe in some strong interaction sites generated by impurities. The condensation is low as the saturation vapor pressure is very high. The fraction of free molecules which are not trapped or condensated is rather high. The sorption isotherm is lightly curved to the top. If one draws the linear transformation of the isotherm, one will obtain a straight line with a negative slope. In order to perform the agreement with B.E.T. equation, one should remove the number of the free molecules from the total mass absorbed.
Example 1, case of light trapping and low condensation :