How do you evaluate a great disparity in estimates for repairs?

We need to get a new roof on the house. So far, we have two estimates and one is 4 times higher than the other! Both claim to be doing the same things and using the same shingles. The higher one is from a national chain, the lower a local guy we haven’t heard of. We will probably get a third estimate and check out references for the local guy. Obviously, we do not plan to go with the national chain, but why such a disparity?