There are various reasons this could happen. For a small difference I wouldn't worry about it. Who knows how windows is even computing that number?
Given how small the difference is, I suspect some kind of difference in how Windows is allocating space for the files. Even though you say elsewhere that all the block size info you can find is the same, filesystems have gotten very complicated, and there could be other parameters you don't see.
Another possibility would be something like sparse files, if a file has empty space on the old drive that's been turned into zeroes taking up space on the new drive. I would hope the copy tool would handle that correctly but who knows.
It could be that some parameter inside Windows itself has changed in the meantime. For example, how much space it preallocates for storing filenames in an empty folder.