Insurance companies base the replacement cost of a home on several factors - the size of the home, type of construction, features, etc. The larger insurers have huge databases with tons of variables that give them an idea of how much it would cost to rebuild your home if it burned to the ground. When applying for a homeowner's policy, you will give them detailed info on the home. Better yet, provide them with a copy of your appraisal so they have more info to go off of.
I used to work as an insurance agent, and in shopping for insurance, your goal should be to work with your insurance company to determine the proper amount of coverage for your home. Most policies also include a buffer, called "extended dwelling coverage" that tacks on a bonus amount to your coverage amount to account for miscalculations and cost overruns if the unthinkable happens and your home needs to be rebuilt entirely.
Talk to a couple insurance agents,and ask them to explain the differences in detail and how they came up with their figures. Saving $100 bucks is nothing compared to what it might cost you if you pick the wrong insurance company. Hopefully you never need them, but if you do need to file a claim you want them to bend over backwards to help. Pick the agent that you feel most comforable with and who seems the most competent.
Hope this helps!