This is true for almost everyone who has tried knock detection. You can find a number of discussion threads in the forum if you search for “knock.“ sometimes it’s just the wind shaking the door a little bit.
There isn’t really anything you can do about this except if you are using a sensor which allows you to change the sensitivity, and unfortunately, the smartthings model does not.
A few people have been able to improve things by putting weatherstripping around the door, and you could try that.
If you really need knock detection a better option is to combine it with a second Kind of sensor so you can be more certain there’s a person there. Probably the most reliable would be to put a pressure mat under a welcome mat at the front door. Then only report it as a knock if there is enough weight on the mat at the same time that the vibration is detected.
United security pressure mat
Other people combine it with some kind of motion sensor or camera detection.
But a vibration sensor on its own is just reporting on vibration, regardless of what shook the door.
We had one community member who had a couple of kids and whenever they would run down the hallway past the door it would set off the knock alert.
So it’s just a matter of trial and error to see if it’s reliable enough for your own purposes.