It sounds like your system is more efficient than the software setting. I'm not familiar with BF but I assume it is similar to Beersmith. If so, you can gradually increase the Brewhouse Efficiency. To eventually get this set up correctly keep taking all measurements (pre boil specific gravity, pre boil volume, post boil specific gravity, post boil volume). You can then adjust the boil off rate with the Brewhouse Efficiency to match your system.
You could have carried on boiling until you hit approx 19l but your OG would have been significantly higher. Again, for next time increase the Brewhouse Efficiency setting and the software should adjust down the amount of malt required.