I can see a few possibilities, but they are strictly guesses on my part.
As I mentioned, PDB is also Palm DataBase, and it would seem that possibly 3 of your files might be dbase files rather than ebooks.
PGA_Champions
-MobileDB.pdb
UK_Football_200
-MobileDB.pdb
Golf_Rules_in_B
-TealInfo.pdb
* /me fires up Google...
MobileDB is definitely a dbase file -- a bit of Googling revealed it's filenames seem to attach the "-MobileDB" onto the end.
Also
TealInfo is another Palm dbase format from
TealPoint. TealPoint also makes a Palm DOC reader called
TealDoc but I feel it is safe to presume they might use TealDoc in the filename as well, rather than TealInfo.
It would appear, then, that Palm Reader has the ability to distinguish an ebook pdb from a dbase pdb, if my guesses turn out to be correct.
So, removing the 3 databases from the list would leave you with 2 actual ebooks, and Palm Reader agrees with that.