Some nes files are being zipped by Romcenter, why?

[mundodisco]

Hi everyone,

I noticed while checking my NES roms, that Romcenter zips these five roms:
Mega Man (World) (Mega Man Legacy Collection)
Mega Man 3 (World) (Mega Man Legacy Collection)
Mega Man 4 (World) (Mega Man Legacy Collection)
Mega Man 5 (World) (Mega Man Legacy Collection)
Mega Man 6 (World) (Mega Man Legacy Collection)
each one with just one *.nes file inside with the same name of the zip.
The other 3001 NES roms are left unzipped, as it should.
If I uncompress those 5 files and recheck with Romcenter, Romcenter "fixes them" compressing them again.
I am using the official NES dat file from Dat-o-Matic, and I cannot find inside the dat file, anything special about those 5 roms indicating that they should be zipped.
Can anyone think of a logical reason for this behaviour?

Kinds regards,
Gabriel