I honestly don't know if this is a bug or not, seems to me like it probably is tough as I assume you shouldn't be able to get over 100% at all.
I don't know how this happned, but I can tell you I did NOT first reach 100% and then teleport to hearthfire 15 times. I got this from using my roads.
And when I say I don't know how this happned I mean it. I just logged in and it was like this, from what I remember I had somewhere around 85% when I logged out near a resource node. Logged back in today and it was 115%.