Different crushes and different grains used on different systems absorb varying amounts of water. Since your amount of wort pre and post boil were both high perhaps your system/pot/kettle loses less water through boil off than others. That you still ended up with a 1.068 with about 4 gallons is a good thing. A .002 give or take is WNL of variances... and not a big deal. Kits are rarely spot on, fortunately yours erred on the plus side. Sounds like you did a good job. Should be a good tasting beer...