Effect of orbital shape (l value) on orbital energy
        (Penetration Effect)

The radial probability distributions (RPD) for 2s and 2p orbitals are given in the figure. Note that although the major lobe of the RPD for the 2s orbital is at a larger r value on average than the RPD for the 2p orbital, a small lobe appears close to the nucleus.

 
This means that the 2s electron spends part of its time "penetrating" close to the nucleus. This increases the attraction of the nucleus for the 2s electron and correspondingly lowers its energy (makes it more negative) compared to the 2p orbital. Generally, for a given n value, the lower the l value, the lower the subshell energy level: s<p<d<f.