By what principle does winrar split the archive into parts?

Question

Data archiving

There are 5 parts of the archive. Unpack the 1st into folder A - ok. Unpack the 2nd folder into folder A - duplicates. If you unpack all parts of the archive at once into folder A - ok, ~ 13 thousand files. If you open parts of the archive one by one, then the sum of the files is ~ 40 thousand.
What is the principle of the parts?