This is compressing video recordings of a computer desktop. The frames don't change much so this isn't surprising.
Can you clarify this please? Does h.265 have some better deduplication compression or does reencoding apply something that the original recording could not, meaning h.264 would have the same benefit.
Reencoding the same input file as h264 again but using FFmpeg defaults would probably yield similar file size reductions.
This is just a bogus post without knowing what parameters were used to encode the original h264.