I had planned and thought I ran 13.1, as reported on my 245 music. After reviewing the 1 mile split times, mile 8 was only 0.3 miles. It looks like it counted that split inaccurately as a mile. (Or inaccurately went to mile 9 only after 0.3 completed on mile 8). You can see the short mile 8 while checking the map view also.
The watch/software glitch/issue was likely triggered by a brief pause/resume around the point of the short logged mile 8.
Need accuracy is tracking/reporting distance during a run so this should be understood and resolved.