Are you taking a full backup? Because mostly, PC's that have a total size of x GB also have some hidden partitions. We do backup those partitions also as part of a full backup since some of them are necessary to do a full recovery. Considering the difference in size, I might suspect that you are also have a recovery partition (which is hidden) that you are protecting also.
If you go to the wizard, and select volume backup, you will be able to see all the partitions
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