Because each of the three credit bureaus records and reports information about you differently, you have different credit histories for each. Since your FICO Score is based on that history, you have different scores for each bureau.
By getting scores and reports for all three bureaus, you can check for errors and know where your credit stands regardless of which score your lender uses. The one-bureau credit report or plan will only provide you with access to one bureau instead of all three bureaus, so you won't get a chance to compare your information.