I guess this is really a Windows question, and I almost understand Windows.
But really, I'm just guessing.
The partition table, which is at the beginning of the drive and not in any partition, describes the sizes and locations of the partitions. But besides that, each partition has to be formatted for its size and for the appropriate OS. Formatted means (among other things) that the FAT (file allocation table), which is inside the partition, has to account for all the space in the partition, so it can allocate files anywhere inside it.
Now, the partitioning tool supplied with the Linux installer is designed to shrink Windows partitions to make room for Linux. In doing so, it has to adjust the FAT, otherwise Windows would try to overwrite the Linux partitions. But (I'm wildly guessing here) it might not have been properly designed to unshrink Windows partitions. It looks as though the partition table (which is what fdisk works with) shows the larger partition, but the FAT inside the partition hasn't been readjusted to work with the whole space.
Of course, in theory, you can back up the partition, reformat it, and restore it. Partition Magic is a Windows-compatible utility that is designed to change the sizes and positions of Windows (and some other) partitions, reformatting as necessary, without affecting the files inside. I've used it, and it works, but it costs money. I've heard of other, less commercial utilities that do the same thing, but never tried them.