different behavior for --backup-dir relative/path vs --backup-dir /full/path

[duane at blur.com]

Why does using --delete -b --backup-dir relative/path
puts the deleted files in the target folder, 
while using the full path works as expected?

my experimentation...

root at feral:/tmp/test
(0)>uname -a
Linux feral 2.4.22-1.2115.nptlsmp #1 SMP Wed Oct 29 15:30:09 EST 2003 i686 i686 i386 GNU/Linux

root at feral:/tmp/test
(0)>rsync --version
rsync  version 2.5.6  protocol version 26

root at feral:/tmp/test
(0)>tree
.
|-- test0
|-- test1
|   |-- a
|   |-- b
|   `-- c
`-- test2

root at feral:/tmp/test
(0)>rsync -avv --delete -b --backup-dir test2 test1/ test0
building file list ...
expand file_list to 4000 bytes, did move
done
expand file_list to 4000 bytes, did move
deleting in .
delta-transmission disabled for local transfer or --whole-file
./
a/
b/
c/
total: matches=0  tag_hits=0  false_alarms=0 data=0
wrote 77 bytes  read 20 bytes  194.00 bytes/sec
total size is 0  speedup is 0.00

root at feral:/tmp/test
(0)>tree
.
|-- test0
|   |-- a
|   |-- b
|   `-- c
|-- test1
|   |-- a
|   |-- b
|   `-- c
`-- test2

9 directories, 0 files

(0)>mv test1/c test1/d
root at feral:/tmp/test

(0)>rsync -avv --delete -b --backup-dir test2 test1/ test0
building file list ...
expand file_list to 4000 bytes, did move
done
expand file_list to 4000 bytes, did move
deleting in .
backup_dir is test2
keep_backup c -> test2/c
delta-transmission disabled for local transfer or --whole-file
./
d/
total: matches=0  tag_hits=0  false_alarms=0 data=0
wrote 77 bytes  read 20 bytes  194.00 bytes/sec
total size is 0  speedup is 0.00
root at feral:/tmp/test
(0)>tree
.
|-- test0
|   |-- a
|   |-- b
|   |-- d
|   `-- test2  <=== why here if relative path?
|       `-- c
|-- test1
|   |-- a
|   |-- b
|   `-- d
`-- test2      <=== goes here if full path given

11 directories, 0 files