I've made several kits in my lifetime & some come with just barely enough fabric & others come with lots of extra. I've never made a BOM so I can't really comment on your issue. However, when I've ordered kits from Connecting Threads for things like table runners I always seem to have a lot of extra fabric left over.
Since you are hoping to use the leftovers for borders can't you just set aside the left overs until it is border time & if you haven't needed them by then I would say it's safe to go ahead & make the borders with the leftovers (since borders are one of the last things you make on the quilt top).