All kinds of instrumental testing methods have been attempted to substitute for sensory analysis of fabric softness, however, the prediction performance is poor. This alone necessitates a deep investigation and understanding of the difference between sensory analysis and instrumental testing under the comparable constrained conditions. By establishing an equivalent biomechanical model to the handling manner in which human fingers sense lateral compression properties of fabric, the mechanistic principle of sensory evaluation on fabric softness property is explored and compared with instrumental testing principle by two proposed indexes, namely the mechanical sensitivity of touch receptors and the sensory sensitivity of human tactile system. The results show that the mechanical sensitivity of human touch receptors leads to the intrinsic difference of sensory analysis from instrumental test on fabric softness by lateral compression. The mechanical sensitivity monotonously changes with the scaling coefficients and the stiffness coefficients of quasi-static and nonlinear compression deformation characterizing both fabric and human skin. The sensory sensitivity depends on both the mechanical sensitivity and the ability of human central system to interpret the information from the peripheral nervous system. The conclusion is that the two-level sensitivity leads to the intrinsic difference between sensory analysis and instrumental evaluation on fabric softness by lateral compression.