See:
https://secure.phabricator.com/D12451?vs=30366&id=30367&whitespace=ignore-most#toc

That view shows the diff between the last two diffs on that revision (30366 and 30367). Specifically it shows adding a new line "What goes between A and B" and removing an underscore from "B.0".

Click on Download Raw Diff which results in a URL like:
https://p.phcdn.net/file/data/mkgo4mrctv365hprxmxx/PHID-FILE-dzxe4lxgkkdbeouj3xsp/gma4egf6wtyrjgfc/D12451.vs30366.id30367.whitespaceignore-most.diff

The URL contains the (vs30366.id30367) which I interpreted that it should show the diff between those two diffs. The resulting raw diff appears to be the raw diff of the Base diff to the last diff, specifically:

diff --git a/manifest.xml b/manifest.xml
new file mode 100644
--- /dev/null
+++ b/manifest.xml
@@ -0,0 +1,7 @@
+{
+	"name" : "aroo"
+	"data" : {
+		lines : [],
+		alpha : "x.x1"
+	}
+}
diff --git a/speck1.txt b/speck1.txt
--- a/speck1.txt
+++ b/speck1.txt
@@ -1,4 +1,5 @@
 123
+ONETWOTHREE
 A
 What goes between A and B
 B.0

Which does not show the "What goes between A and B" modification.

I'm not entirely sure what should be showing in the diff view or the raw diff download. Additionally something that I find confusing is that on that diff view (between last two diffs) the maniphest.xml file says "This file was added." followed by "This is an empty file. Show File Contents".