Some of your creditors and lenders might report only to one of the credit bureaus. And, since credit bureaus don’t typically share information, it’s possible to have different information on each of your reports. Ordering all three reports will give you a complete view of your credit history and let you repair your credit at all three bureaus instead of just one.
If you find information that is incorrect, you can file a dispute. Remember too, that items on your credit report that you don't recognize could also be potential signs of fraudulent activity — someone working to secure credit in your name for their own use. Make sure you're clear on items that could potentially be fraudulent, versus those that may simply be inaccurate.