Good Morning and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
Yes, I do know what is causing thsi. In older cats, they will have overactive Sebaceous Glands. These are oil glands within the hair follicles that produce this oil causingthis matting. It is called Stud Tail in some male cats and there is no way to stop this.
The best treatment for this is to bath Milly once a week...if she does not like this, use a waterless shampoo available at the pet store. You will have to cut out the matts using a comb under the matt to be sure you do not cut her. Then brush her daily to keep the matts from forming.
This LINK may give you more information. Even though this refers to males, it is the same thing happening to Milly. It is not hurting her but with some treatment, you can get this under control.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, Dr. Loretta