DisplayName property is supposed to reflect the original file name value (as present in the email data), while FileName should be "safe" version of it (meaning it's a valid file name and not a duplicate).
So based on your description, it looks like the first mail really did contain image.jpg, image(1).jpg and image(2).jpg files, while the other mail might not be processed by Rebex Mail properly.
We'll look into this. An issue like this could easily got overlooked because TNEF/winmail.dat mail messages have been quite rare for years. If you could share a sample of these emails, it would be helpful. Thanks for letting us know about this!