Like ame said, labs besides GIA or AGS tend to grade more loosely. Sometimes their grades come close to reality, and sometimes they are comically far off. What a person needs to keep in mind is that you aren't getting a deal by purchasing a diamond graded by a lab like IGI or EGL. People selling diamonds know that a diamond graded by GIA can fetch a higher price; if they know it can get a good grade from GIA, they will have it graded there and charge a higher price. They know that other labs grade more loosely, so they often send lesser quality stones to those labs to get inflated grades, but they still sell for less, even with a "good" report from those labs. There really isn't much of a way around the fact that you get what you pay for in diamonds, and a GIA report is valued because it is regarded as accurate, whereas other labs may or may not be.
Obviously, the most important grade a diamond can get is your approval, at least if you are buying the stone for yourself. So if the stone performs better than any other you have seen and you love it and it is in your price range, then it might be worth the price. Just be aware that its certificate may be off by several grades.