Am i right in saying that that is a natural beach gravel?
If so then the chances are that it is calciferous which will buffer your KH up. I think i'm right in saying that it will also buffer up your GH and PH.
There are a few "chemists" on here so they will confirm but i think i'm right?
Yes it's your gravel that it raising your KH. Just keep an eye on it to make sure it doesn't get too high. 7 is OK but 15 is starting to get high IMO. Regular water changes will help to keep it down. Better to do small changes more often rather than big changes less often. This is to reduce shock for the fish. You may find it settles down a bit with time. A rise in KH will also raise the pH. The GH may also rise but this shouldn't be too much of an issue compared to the KH.
Your bogwood actually lowers KH so it is a good idea to have plenty to counteract the gravel.