If the "port range" is left empty, a default port range is used. This is a special port combination that has all the "known" ports, excluding ports we know to be problematic (for example, printer crash when port scanned on certain ports). The total is about 2,000 ports that we believe cover all the interesting services while being safe enough to scan.
You can change the range to something like: 1-1024 (to only scan the first 1024 ports) 1-65535 (to scan everything. This may be potentially destructive when running an internal scan and will also be much slower than our regular scan) Or you can use any other combination such as: 22,25,80,443