It's not always about pure frequencies.
• Could be that the LN digraph is more common than the HN one for instance.
• Also, the H is kept in its QWERTY position rather than moving it to gain little benefit (as is the A for instance).
• Also, it's nice for people from other countries that the H is kept on the home row because it's quite common.
• Also, it may be that your effort is higher than others' for these stretches. Do you have short fingers? Do you keep your wrists straight while typing?
H and D are the least common of the home row characters along with R. L and U are of similar frequency as them. Their frequency differences alone aren't so different that placement should be based on that alone.
And yeah, Shai is the one to ask for the full story. ;)