Assume I click on "preview" button in Scangear.
Some paper is scanned with 150dpi.
Then I select a crop rectangle and clicked "Scan".
ScanGear re-scans then again the source.
Why?
Other scanner software uses the stuff from the preview scan process phase to immediately create the target file (PNG or PDF).
A re-scan is unnecessary.
ScanGear has nothing to do with PDF24. PDF24 just uses the scanner drivers in your system and if the driver rescans, then it is implemented in that driver.