Compression efficiency: some mmCIF files are smaller than their BinaryCIF counterparts when not gzipped

[papillot]

This is a question rather than an issue report. I have downloaded the BinaryCIF file for structure 5z6y from RCSB and the corresponding mmCIF file.

format	size	gzipped
mmCIF	227kB	58kB
BinaryCIF	269kB	32 kB
mmtf	24kB	17kB

What surprises me is that the BinaryCIF file takes more space than the mmCIF file, even if most of the information is contained in the atom_site table which should be amenable to efficient compression.
This seems to contradict the claims of the original BinaryCIF publication.

I am wondering if there is an issue with the current implementation of the format which would use less efficient compression techniques?