Why do you find this odd? That's what you get when a scanner enhances elements by default. It's not perfect, so it doesn't always recognize the elements, so some stay at a lower resolution.
I posted an example of this on another thread. You can take a look at a PDF file of a scanned book and see the scanning process producing the same effect. That example comes from page 89 of the PDF (page 86 of the book) if you care to download the file and inspect it yourself.