Unexpected conflict switching within colo branch

I got a conflict switching from one release of my project to another. I don't expect it to be the case, since one is just a future version of the other.

Also, it's not clear what it would mean to resolve the conflict.

jml@valour:~/src/testtools$ cd /tmp
jml@valour:/tmp$ bzr branch lp:testtools
Branched 245 revisions.
jml@valour:/tmp$ cd testtools/
jml@valour:/tmp/testtools$ bzr switch -b trunk
Tree is up to date at revision 245.
Switched to branch: /tmp/testtools/
jml@valour:/tmp/testtools$ bzr switch -b testtools-0.9.12 -r tag:testtools-0.9.12
Updated to revision 245.
Switched to branch: /tmp/testtools/
jml@valour:/tmp/testtools$ bzr switch -b testtools-0.9.13 -r tag:testtools-0.9.13
Text conflict in NEWS
Text conflict in testtools/__init__.py
Text conflict in testtools/tests/test_deferredruntest.py
Text conflict in testtools/tests/test_fixturesupport.py
Text conflict in testtools/tests/test_testresult.py
Updated to revision 245.
Switched to branch: /tmp/testtools/
jml@valour:/tmp/testtools$