FICO is the company that developed the formula that is used by the three credit bureaus: Experian, Transunion, and Equifax to calculate all your financial and credit transaction activities and transform them into a figure.
When you get your report you will note that the three versions of your file from Experian, Transunion, and Equifax will most likely have different points. Though the three bureaus use the formula developed by FICO, they each have their own unique variations in the way they calculate your score.
Another reason for differences in the figures the three of them attach to their version of your report is because not all creditors furnish the three of them with information of your activities. Some information furnishers may decide to correspond with only one bureau or as it pleases them.
Usually, when you apply for a mortgage loan, your mortgage creditor will purchase copies of your credit report and they will also have the three versions sent to them. What they will do in deciding which of the three to use is a simple one: assuming your report reads: Experian = 590, Transunion = 681, Equifax = 637, they will use the report with the middle score.
If you place the three points in ascending or descending order, one of them will always be in the middle. It doesn't matter which of the agencies that produces the figure, the most important thing is the particular figure that occupies the middle position. This is what is known as the median in arithmetic. In the case above, the middle score which is 637 will be used.
There is something significant about the situation here: Despite the fact that you have a higher score of 681, it wasn't used. If you know this, your next reaction should be to work towards the target of raising your current credit figure.