Well it is probably the unknown as to why Thompson is rated so low.
Don't know about Madden, but Thompson was drafted as an end, while he has a decent burst, to my knowledge has never played linebacker. So, if you are evaluating him as a player their is a great deal of known in how he will convert to a Linebacker.
Looking at the difference between Kampman and Thompson, I don't understand the margin... but Thompson in real life this year is a wild card for sure.
"The oranges are dry; the apples are mealy; and the papayas... I don't know what's going on with the papayas!"