Is it normal for a Properties dialog box "contents" to fluctuate for a very long time?

I have a number of external HDDs. There are two in this example which are the original and its rsync duplicate, and I want to check and compare the file structure.

I have used the contextual menu to show Properties for a number of directories on each HDD, and it has apparently been calculating the contents for a couple of hours.

Interestingly, the Contents quantity and size fluctuates up and down.

For example, one of the volumes will say 18,195 items totalling 10.6GB, then 8,445 items totalling 1.2TB, then 21,237 items totalling 530.2GB, then 4,556 items totalling 1.6GB, and so on, all within 20 seconds.

Ubuntu 14.04