Assume simple change:
Modify file A.txt
Add new file B.txt

When it's sent with arc diff you get new revision with 1 diff attached which shows 1 file added and 1 modified, now when you commit this revision in SVN, Phabricator would parse it and update revision saying that it's now closed, it will also attach a new diff to it (which it got from SVN), but this diff would show both files as "Modified" with no files Added.

Same revision works correctly on top of GIT repo with both diffs showing 1 modify and 1 add. Because of this arc export --diff would export invalid patch files for SVN repo marking files as modified instead of added.