in a pages-table what does a plus sign mean

I have a Pages doc with a table. For some reason, some of the cells in the table have a little plus sign on the right of the cell. The text in those cells is not centered top to bottom. I tried to see what was missing by Showing Invisibles but there were no invisibles to be found. Yet there is a plus sign and the text is not centered. Any ideas? All the cells have the same height, so that's not the issue. Thanks all...

Fred J.