What is the definition of post boil gravity and how is it different from original gravity - or are they both the same thing. Beersmith uses both terms but aside from the fermenter top up to me they seem like they should be the same thing. On my brew the measured post boil gravity was 1.082 yet on the Brew Steps it gave an Est Post Boil Gravity of 1.059. I'm wondering whether it has something to do with the fermenter top off but brewsmith puts post boil volume and post boil gravity beside one another so you'd expect both figures to be occurring at the same step in the process.
I'd have to look at my beersmith to try and figure it out exactly. I can say this, beersmith makes simple stuff too complicated. Postboil gravity and OG are the thing in real world terms. Unless, as you said, you execute some sort of kettle water top off step, or add water to the fermentor. Beersmith doesn't actually indicate when or where the OG reading should be taken, (out of the kettle or from the fermentor).
The second part of your post however is the most troubling. Est post boil gravity issues. I've had plenty of issues with all the estimates that BS tries to throw at you. Unless you have carefully input all the variables in your equipment and mash profiles I have found those numbers can be pretty far off from reality. The best thing I can tell you is that you need to brew a beer in your normal way. Take really good notes of water volumes, wort volumes and gravities along the way. Then go into a clean BS profile, buidl you recipe and play with the variables until BS #s match what you observed in the real world. Beersmith is great, but the learning curve is tough and BS doesn't tell you this upfront. Its pretty frustrating.
You have to take real world data and make beersmith bend to reality not try and bend reality to what beersmith tells you.