error when git-diff get pipe args

$ cat a
111
fdsf
333
$ cat b
111
222
333
$ git diff a b                             # OK

diff --git a/a b/b
index 768560b..641d574 100644
--- a/a
+++ b/b

@@ -1,3 +1,3 @@
 111
-fdsf
+222
 333

$ git diff <(cat a) <(cat b)          # ERROR: no result print out

diff --git a/dev/fd/63 b/dev/fd/62
index 311e262..554e9f4 120000
--- a/dev/fd/63
+++ b/dev/fd/62

@@ -1 +1 @@
-pipe:[729963]

\ No newline at end of file
+pipe:[729965]
\ No newline at end of file

# compare with diff
$ diff -u a b                            # OK

--- a 2013-11-05 00:18:17.394805923 +0800
+++ b 2013-11-05 00:18:25.546805865 +0800

@@ -1,3 +1,3 @@
 111
-fdsf
+222
 333

$ diff -u <(cat a) <(cat b)         # OK: get result in same case

--- /dev/fd/63 2013-11-05 01:02:59.706787117 +0800
+++ /dev/fd/62 2013-11-05 01:02:59.706787117 +0800

@@ -1,3 +1,3 @@
 111
-fdsf
+222
 333