Y/A shows my 4 chosen categories on the upper left still, as always, but presents me with questions from all categories.  I'm signed in, so?

Those are categories you selected to pin so you can move fast to those categories.  This does not stop you from going to any other category or Yahoo showing you a question from any other category.

Thats Because your on the Main Page the Main Page will show you Questions from all Category's if you just want Questions from a Specific Category you have to go to the Category Page to get only those Questions to Pull up

If you're on the main YA page, you will see questions from many different categories. Your 'pinned' categories/subcategories at the top left simply provides you a quick way to go to those areas. It has nothing to do with the topics of the questions you see when you're on the main/home YA page.

A computer algorithm is personalizing which questions it feeds you based on the types of questions you have answered in the past. I have about 8 subcategories pinned above the category list, but I get fed questions from Religion and Mythology, Polls & Surveys and Politics occasionally. I usually do not hang out in those subcategories. The server computers can and do list questions in more than one subcategory or move them based on keywords in your and other people's answers. You may have customized your feed at some time.  

This is not a glitch. I have NEVER claimed to be a computer tech or computer expert or engineer. I've also been on YA for over 11 and a half years. Verizon has gradually fixed many of the eternal glitches YA used to have.