Anyone care to have a guess at why?
There is no difference in size.
1 gigabyte = 1073741824 bytes
19665999979 ÷ 1073741824 = 18.31538973283
Going by the first decimal point, they are the same. It all comes down to how the final number is rounded off. Depending on what system is running that drive, the rounded-off estimate could differ. Both of the rounded figures you show are correct, based on the simple math calculation I have shown. The only difference being how the decimal was rounded.
Why is the rounding showing differently? It's hard to say. From the icon previews you are showing, those are two different folders. These could be two different drives or two different pieces of hardware. There are also two differnt times for each folder. One time it could have rounded to a lesser decimal, the other time by more decimals.
Keep in mind that while the rounded-off estimate for GB is different, the byte count is the same.
That is what is important, not the rounded figure.