Differential image motion monitor (DIMM) is by far the most successful and the most widely used instrument for the measurement of the optical turbulence-induced seeing angle affecting big telescopes. It is actually based upon the hypothesis that refractive index fluctuations in the whole atmosphere follow the Kolmogorov energy cascade turbulence model. However, many in situ measurements show that the the optical turbulence in the atmospheric surface layer can be non-Kolmogorov. In the following is discussed, through a simple and straightforward model, the effect of a non-Kolmogorov optical turbulence, especially in the atmospheric surface layer, on the DIMM seeing angle, and its consequent disagreement with the seeing angle affecting large telescopes.