That's just a way to differentiate various samples of the same variety on the chart.
You will notice however there is a variety called BoroNgoru, which is different than Boroguru, as well as Borogu. All different. In text you may also find spellings like Borogoro or Borogoru, but they aren't necessarily different cultivars, just different regional names from the country of origin.
Also, one persons Boroguru isn't always exactly identical to the next persons Boroguru....just a rough guide.