Look at Mormons. They have a rule that you have to donate 10% of your income. If you don't than you aren't pleasing god and god might punish you.
In reality the average Mormon doesn't donate 10% but might feel guilty for not doing so. If someone who would donate 7% would know that they donate above average, they would feel less guilty about not meeting the goal of donating 10%.
Sure, but why 10%? Why not 15%? Or 20%?
It is possible that they are setting the bar too low. You might have many people who would have given 30% had not the command been for 10%, but for 30%?
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.