We study the frequency distributions of Delta Scuti stars observed by the Kepler satellite in short-cadence mode. To minimize errors in the estimated stellar parameters, we divided the instability strip into ten regions and determined the mean frequency distribution in each region. We confirm that the presence of low frequencies is a property of all Delta Scuti stars, rendering meaningless the concept of Delta Sct/Gamma Dor hybrids. We obtained the true distribution of equatorial rotational velocities in each region and calculated the frequency distributions predicted by pulsation models, taking into account rotational splitting of the frequencies. We confirm that rotation cannot account for the presence of low frequencies. We calculated a large variety of standard pulsation models with different metal and helium abundances, but were unable to obtain unstable low-frequency modes driven by the kappa mechanism in any model. We also constructed models with modified opacities in the envelope. Increasing the opacity at a temperature log T = 5.06 by a factor of two does lead to instability of low-degree modes at low frequencies, but also decreases the frequency range of Delta Sct-type pulsations to some extent. We also re-affirm the fact that less than half of the stars in the Delta Sct instability strip have pulsations detectable by Kepler. We also point out the huge variety of frequency patterns in stars with roughly similar parameters, suggesting that nonlinearity is an important factor in Delta Sct pulsations.

11 pages, 7 figures. 1 table