The cause is structural. A PDF is a stack of graphical objects, not a flat image. The rectangle you drew becomes one more object on top of the text object, while the text itself stays untouched in the page's content stream. Dragging a selection, Ctrl+C, any extractor, or a plain document search returns the original content. Courts, ministries, and large corporations have all been caught by this – stories about 'blacked-out' but perfectly readable documents resurface every year.
Genuine redaction works differently: the fragment is physically removed from the page content rather than covered. Afterwards there is nothing beneath the black area – no glyphs, no coordinates, no way to recover the text. That is the fundamental difference between painting over and cutting out.
A few practical rules. Verify the result: open the finished file and try to select and copy the blacked-out area – it should come back empty. Remember scans: if the document was OCR'd, the text lives in an invisible layer that must be redacted too. And do not overlook metadata and hidden layers – data escapes from more than the visible part of the page.