Even if you've set up an accurate equipment profile in BF, differences are pretty normal. Some common reasons are: The recipe used a different model for calculating IBUs, it's an older recipe and doesn't figure utilization for steeping/whirlpool, it doesn't give the AA% for the hops being used or you didn't change the AA%s to match those of your hops. There are other reasons too, that include differences in equipment and procedure as well as errors in the original recipe.
If you feel that the calculated IBUs in your own recipes are pretty accurate, adjust those in others' recipes using your numbers. Your other alternative is to use the amounts cited in the recipe and adjust the AA%s accordingly.