It's spam. They burst out with 800+ at the start then it cools down to whatever. The difference being is that most average players (such as myself) force the spam where as when professionals play, it's second nature to them (they do it without thinking about it). It enables them to be faster without paying any kind of attention nor effort to their speed (even if it is spam).
NGC posted a documentary on the difference between observational (average player) vs instinctual (professional player) play habits. It's very old (as in around the 2nd - 3rd generation of SC pros) but still a good watch