Bug with rewards?

So if I have a set of rewards as follows:

$10
$20
$30
$40

If I click the radio button next to $10, then $10 will be automatically entered into the donation amount box. Then if I click on $30 the donation box will update with $30. Now if I click back to $10, it will not update but will remain $30. Clicking to $40, will bring it up to $40 in the donation box.
Any step up is updated but any step down is not. It may give the appearance of being shady.