Requirements on material properties for extrusion-based additive manufacturing mostly focus on the rheological behavior of the cementitious material being printed. The layer interface strength is therefore often considered to result from a proper mixing or remixing of two consecutive layers induced by the deposition process itself and therefore from the material thixotropic behavior. We show however here that, in the case of smooth interfaces, the drop in interface strength finds its origin in the water evaporation from the free surface occurring during the short time interval between two successive layers. Our results and their analysis within the framework of drying physics suggest that the water loss is localized in a dry region at the free surface leading to an incomplete cement hydration and high local porosity. We moreover compare here various experimental protocols allowing for the assessment of a drop in bond strength.