Compressed PNG files?

[Raccoon Sam]

Looking at box2-open.png with a hex editor, the first $12 bytes look like this:
89 50 5F A2 0D 0A 0B EF 00 00 11 E8 49 48 55 B7 00 00 11 AE
A standard PNG should look like this:
89 50 4E 47 0D 0A 1A 0A 00 00 00 0D 49 48 44 52 00 00 00 AF

Do you see what's wrong?
Every other word is wrong, every other word is right. I bolded them for you:
__compressed: 89 50 5F A2 0D 0A 0B EF 00 00 11 E8 49 48 55 B7 00 00 11 AE
uncompressed: 89 50 4E 47 0D 0A 1A 0A 00 00 00 0D 49 48 44 52 00 00 00 AF

Very peculiar. That's the only lead I've got right now.

EDIT: If you wish for more help/insight, it's always wise to include as much as files as possible. Excluding obvious false positives like "MUSIC.DAT" or something, you should try uploading _all_ the PNGs and any other suspicious file that might reside in the filesystem.
EDIT: The nibbles of every compressed word's first byte are always the uncompressed one's counterpart ±1!
EDIT: The second nibbles of every compressed word's second byte are always the uncompressed one's counterpart ±5!