Gravity Question - What did I do?

hey everyone.

this was my fist entirely at home brew, the recipe is listed below here. everything went relatively smoothly and i think the beer will turn out just fine. i have one question about the gravity however.. BS estimated 1.048 for my final but i measured it at 1.040 at 80F. even assuming a .003 differential for the temp im at 1.043 and a bit low. unless im calculating that adjustment wrong the only thing i can imagine is that my gravity dropped because i added too much to start to account for my boil off.

i boiled in a 10 gallon kettle, using 6 gallons to assume a one gallon evaporation. i actually had a 1.5 gallon loss and didnt bother to add more because of the low OG.

is that a possible explanation? and if so, am i better off just boiling 5 gallons of water then adding more at the end to make sure im closer to my target? thanks for any insight

Brew Type: Extract
Style: Hefeweizen/American Wheat
Batch Size: 5.00 gal
Boil Volume: 6.00 gal
Boil Time: 60 min

Steep 1.00 lb Caramel/Crystal Malt - 10L (11.0 SRM)
Estimated Pre-boil Gravity is: 1.042 SG with all grains/extracts added
60 min 5.75 lb Wheat Liquid Extract (8.0 SRM)Extract
60 min 0.50 oz Saaz [3.60 %] (60 min)Hops
30 min 0.50 oz Saaz [3.60 %] (30 min)Hops
10 min 1.00 oz Saaz [3.60 %] (10 min)Hops
5 min 0.50 oz Coriander Seed (Boil 5.0 min)Misc
1 Pkgs Hefeweizen IV Ale (White Labs #WLP380) Yeast-Wheat

Original Gravity: (Estimate: 1.048 SG)
Final Gravity: (Estimate: 1.011 SG)

Actually with a lower final volume your OG should have been higher. My guess here would be that somehow you had less extract than you thought or your final volume is higher than 5 gallons.