The oddities of the bigram "ed"

[oshfdk]

If VMS was a cipher and different cipher tables (keys, mappings) were used for different folios, this behavior of ed would be easy to explain. Say, for some folios ed maps to plaintext ST, for other folios ed maps to plaintext GP, very different statistics. However, even under this scenario it's hard to explain why it's only ed that behaves like this.

Is it possible to probe trigrams/tetragrams and see whether any particular longer ngram that contains ed causes this effect?