One of the word count InDesign features is that InDesign statistics include overset text. Overset text appears when a text box is too small for the text content. The result is missing words, paragraphs, or even entire pages in a document.
Thus, the extra text extends beyond the visible text frame. Simply put, we do not see text that does not fit into the frame. InDesign shows a small icon with a red plus symbol to indicate that there is an overset text.