We call this "over-accommodation" and it is designed to provide you with the funds you may need for upgrades, cost overruns, etc. We know that home improvement projects can change as they progress. For that reason, the funding banks in the GreenSky Program have credit rules designed to accommodate the amount you requested and, if you qualify, a buffer above that amount.
Regardless of your approved credit limit, however, you are only obligated for the payments you authorize your contractor/merchant to take from your GreenSky Program loan account. The additional credit limit is simply a convenience.