Your picture is a little fuzzy, but it looks like you don't have XML highlighting enabled. Is that correct? If you don't, and are just editing plain text, it's going to spell check the entire document. When editing plain text, the entire document is spell checked. When a programming language or markup language is edited, only comments are spell checked. RSTA can't tell just by the content what language is being edited, it goes by the syntax style.
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