  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.
