They aren’t “off” in time deliberately, they’re just every 2 seconds apart, so they could be before or after a scene cut. Is that what you’re seeing?
Some clients can only scrub to 10 second intervals anyway. The web client is one of the most granular, but I don’t know if it can do smaller intervals.
I’ve never tried generating them more frequently. You could change it to 1 second and test. That would make the preview files twice as big and use twice as much space.
