Ordinary Web
This shows the log of three rounds. As you can see in the first round the DC is what it should be but then from the second round it drops by one and continues like that. Also, the spell actually casts two webs, not one, both of which appear on the list of spells to dismiss.
Web by Greater Shadow Conjuration
Again three rounds, with the correct DC on the first round but then dropping; this time by 3.
This also puts two spells on the list, but one of them just an ordinary web. Somehow.