It's to do with how large/complex the molecules making up the carb are. The most basic, simplest sugar is glucose. More complex carbs like potatoes, rice etc. are basically lots of glucose molecules stitched together chemically into larger molecules which then get broken down by the digestive system into the simplest sugar, glucose.
In the example you've given, it means 2.6 grams are simpler, shorter chain molecules and the rest are more complex, longer chain molecules.
It makes a big difference for people injecting insulin. Insulin isn't just fire-and-forget. It operates over a period of about 3 to 5 hours. Some complex carbs match that time pattern very well. With me, for example, the rate at which brown rice digests will actually stabilise my levels.
Whereas if my bg is dropping like a stone, I'm not going to want to wait a while as I'm not that keen on being unconscious, so shorter chain stuff like glucose which really doesn't need to be digested at all, will get into my bloodstream pronto.