The vertical scrollbars in most editors shows the position of the visible text within the document. The position of the scrollbars is determined by the number of lines in the file. The number of lines in the file can only be calculated by scanning through the file.

Because it is not possible to scan a source document from start to end (imagine how long it would take to do on a 10 TB file), it is impossible to calculate the number of lines in the file, so its not possible to accurately determine the position in the file. Normally these discrepancies are not noticeable, but on some extreme (edge) cases they can yield slightly odd results, for example if you have a 10 MB file, and the first 9MB is on a single line, and the last 1MB contains just one char per line, then the scrolling will be weighted very much towards the start of the file.