2 instances different number of results

i have a NAS. on the NAS i have one instanse of Ubooquity on a docker with the book folder as a volume.

the second is running on ubuntu using openjava, and the folder is mounted via NFS.

one shows 28000 books, the other 13000.

i checked if it is ownership, file permission (644 and 666), i cannot understand why i have different results on the same folder.

any ideas?