PAX is computing the TAR checksum wrong. The standard says that
the checksum needs to be computed over the whole 512 bytes of the
header. PAX only computes the checksum for 500 bytes which is 
incorrect.

To verify the problem try:

	pax -itv < not_a_tar_file

And see what's happening.

Any correct TAR implementation will correctly state that the checksum is
wrong and not try to interpret the data as TAR archive.

To understand what's happening, have a look into the file 'not_a_tar_file',
it is a plain text file.....