If one of the list expands ( > n/5), then the other one shrinks ( < 4n/5) which results in more balanced tree. So T(n) is less in this case.
Here the worst case is when one of the list is of size exactly n/5 & the other exactly 4n/5 resulting in skewed tree. So comparatively T(n) is maximum in this case.
So overall T(n) ≤ T(n/5) + T(4n/5) + n