Can the height of a binary search tree be less than that of a red-black tree?
The fact that a red-black tree is balanced just means that if it contains $n$ nodes then its depth is $O(\log n)$. There is no requirement for the depth to be optimal (i.e. $\lceil \log_2 (n+1) \rceil-1$), only for it to be optimal up to constants. Therefore it is definitely possible for a red-black tree to be deeper than some other binary search tree.

The advantage of balanced trees is that they are guaranteed to be balanced even after operations on the data structure. Since the running time of operations is usually linear in the depth, it is enough for the depth to be $O(\log n)$ rather than the optimal $\approx \log_2 n$.