Why is there 5GB of unallocated space on my disk on Windows 10 machine?

The partitions on disks 1 and 2 need to be the same size because you created a striped (RAID 0) volume. One drive is a 250 GB drive, the other one a 256 GB drive, hence the size difference.

If you don’t need RAID 0 performance, you can also create a spanned volume. It has no special characteristics except that it can be backed by multiple non-contiguous extents of drive space.

They're configured as a Striped Disk (like a software RAID 0 more or less) so in order to do that both disks need to have the same size. It's always adjusted to the size of the less capacity disk. You can't remove neither resize that unallocated space until you decide to remove the Striped configuration. As to why that difference in size... Well, that could be because are from different vendors and therefore not "exactly" the same size.