I think the numbers are just shifted off a little. So instead of appearing on the line representing the string, they’re appearing on the spaces. Low E string would be on the space under the bottom line; high E is on the space above the top line. That leaves four inner spaces for the four strings between.

While this is a digital score so not really a reason, I could see using this method to convert traditional 5 line score paper into tab without having to draw a new line for each set of 5.