I could very well be wrong, but I seem to recall that there is an upper limit on the Trap Affinity skill, and if you exceed it, it doesn't go any higher. You might in fact have 9 Trap Affinity, and -1 keeps you at the maximum of 8. (If that is the maximum.) I recall encountering the limit when I played a Tinker with quite a few +TA items. Do you have other Trap Affinity items equipped? Try taking them off before you put on the boots and see if the number changes.
That is what I thought after posting. Since i went smithing and tinkering, I have a lot of passive Trap affinity. So that is most likely the case.
You should be able to look at all of your skills and items and add up the Trap Affinity. If it's higher than 8, then we know this is the reason.