When the program gets installed it looks at the DLL's on your system and copies in any versions that it shipped with that are newer or that you do not have. There is no guarantee that newer is better or that it is even accurate. When you install a second program it will do the same with the DLL's. No guarantee that the first and second program will even work with each other's DLL's. They are likely to share some DLL's.
Software quits working for inexplicable reasons and programs, which used to work, crash.
An example of DLL Rot is the computer that I am sending this e-mail to you on. For some reason Microsoft's installer program no longer works on this computer. I can't install Office 2000. This is only a minor irritant to me because I have 9 computers in my office that I use. Maybe this will help you or somebody else that you pass the information on to.