Chances are the discrepancy is the result of a partition- or filesystem-resizing operation that changed one of those things but not the other.
To elaborate, partitions are simple data structures that define a range of sectors on the disk that may be used for a particular purpose. Filesystems, OTOH, are much more complex data structures that help you store files. Filesystems are often stored on partitions, and partitions often hold filesystems, but this linkage isn't absolute. A partition, by its nature, defines a size -- 81,920,000 sectors, in your case. Filesystems also have sizes, which are defined independently of the partition -- 78,124,992 sectors, in your case. The two can get out of whack if a partition is resized without resizing the contained filesystem or if a filesystem is resized without resizing the containing partition. Tools to do both tasks exist in Linux, although some tools create or resize both partitions and filesystems simultaneously.
I'm not 100% positive, but I think that the following command, typed in Linux as root or using sudo, should fix the discrepancy:
ntfsresize /dev/sda1 This command resizes NTFS. The ntfsresize man page is a little unclear, but these tools usually resize to the size of the containing partition if no size is specified.
Note, however, that resizing filesystems is inherently dangerous. Given that there's only about a 5% discrepancy, and it's of a filesystem that's smaller than its partition, it's probably safer to just leave it be. A filesystem that's smaller than its partition poses no danger to the system; you're just losing a little disk space. If the filesystem were larger than its partition, that would pose a risk of data loss, but that's not the case for you.