Removing a page from a Word document is harder than it sounds, because a Word file has no pages. It has a stream of content that a word processor breaks into pages while it lays the document out, and different programs break it in different places. That is why the same file can be four pages in Word and three somewhere else.
So this tool does not delete whatever happens to land on a page number. It looks for the thing in the document that made the page begin — a manual page break, a section that starts on a new sheet — and removes that. Every removal is then checked by laying the document out again and comparing it against the original, page by page.
It is built for blank pages you want gone: a stray page break, an empty sheet at the end of a chapter. Where a blank page exists only because the text happened to reflow onto it, there is nothing in the file to remove, and the tool says so rather than guessing.
Word paginates using the fonts, margins and settings available on the machine displaying it. A different environment can break the same content across pages slightly differently, which shifts page numbers.
The file is uploaded to KDP Tools because the processing happens on the server. Both your uploaded document and the generated output are deleted from the server as part of the same request that produces your download. That deletion runs in the normal path rather than being guaranteed, so avoid uploading material you could not risk transmitting.
Removing content reflows everything after it, so headers, section breaks and anything positioned relative to a page can move. This is why the result should be read rather than assumed correct.