First BIAB all grain!

I did my first AG BIAB today and I have to say, it went pretty good! :rockin:
I am a little confused on the efficiency calculations though, I had a pre boil of 1.039 and a post boil of 1.050, the recipe called for 1.042-1.046.
sooo...
1. why did my readings go up after the boil
2. I used this calculator: http://www.brewersfriend.com/brewhouse-efficiency/ and it says I had a 78% efficiency, but it took the pre boil reading, why wouldn't it use the post boil reading?

all-in-all I am pretty happy with my results, and now I will never look back at extract brewing again! (I think it was actually easier to do the BIAB!)

oh, the recipe is Midwest's Liberty Cream Ale, and it sure was nice to have a buddy stop by while I was brewing, and have a home brew with me. (it was his first home brew and I think he is hooked!)

Now that you are all grain you MUST read Designing Great Beers by Ray Daniels. It will greatly enhance your brewing knowledge.
Your reading went up after the boil because of the evaporation during the boil. You concentrated the sugars which will lead to a higher reading.
Sounds like you did everything correctly. Once you get a few more brews on your system, you will have a good understanding of the efficiency you can expect, and adjust your recipe accordingly.

Oh. And also, it doesn't matter what reading it uses since your gravity units will be constant.
If you multiply the pre boil volume by 39 (your stated pre boil GU) it should equal your post boil volume times 50 ( your post boil GU).