This is a self-priming contractors style pump. The curve clearly shows the suggested operating range - however being what it is and how this type of pump is quite often used (throw a suction hose in the sump and start pumping) min / max flow is usually never considered.
Under thus type of use and depending on discharge and suction head, the pump will / could run out along the curve until it cavitates - now it might hunt up and down the curve or conditions might just coincide with the curve and the unit might run OK. At the other end of the curve as the sump may empty, the pump will be operating at very low flow well up the curve and the pump will probably lose prime and snore until the sump level inreases and the sequence recommences.
If you would like to explain fully what you are doing / expect when you are likely to get better advice.
Edited:; the comment re. Operating up the curve is only applicable if you are pumping against a high static discharge head.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)