Highlight tool is inconsistent

I have a customer (a Windows 10 64bit user) who uses Adobe Acrobat XI Professional to highlight relevant content in pdf documents. She finds that when she highlights some documents, instead of selecting the text by line the highlighting the tool will draw an oval shape over a section of the document instead.

I have tested this application on two other machines with different OSes and achieved exactly the same result using the same problematic documents. When some documents in her store are highlighted, they will select text by line, but others will be highlighted by oval sections.

I'm now asking the following questions in this forum because you are the experts who created the standard in the first place: Are these problems caused by the fact that some of the documents in question started their life as scanned images instead of as PDF-formatted documents? Or is there something else at play here? My customer believes that others are able to do the same thing with the same document. I'm trying to be polite and avoid making "prove" her claim by going to the experts first! I hope you will answer these questions because this particular user is a practicing attorney for whom information from trusted validated sources is all that will be accepted!