strange result of `git describe` while bisecting

Hi,

I am bisecting the kernel tree between v3.17 and v3.18, and 'git describe' is used by the kernel compilation process. Why do I get a version v3.17-rc7-1626-ga4b4a2b, that seems outside of [v3.17..v3.18] ?

To reproduce :

git bisect start
git bisect bad v3.18
git bisect good v3.17
Bisecting: 6197 revisions left to test after this (roughly 13 steps)
[754c780953397dd5ee5191b7b3ca67e09088ce7a] Merge branch 'for-v3.18' of git://git.linaro.org/people/mszyprowski/linux-dma-mapping

git describe v3.17-6163-g754c780

git bisect bad
Bisecting: 3086 revisions left to test after this (roughly 12 steps)
[4a4743e840d06a5772be7c21110807165c5b3c9f] Merge tag 'fixes-nc-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/arm/arm-soc

git describe v3.17-3076-g4a4743e

git bisect good
Bisecting: 1506 revisions left to test after this (roughly 11 steps)
[a4b4a2b7f98a45c71a906b1126cabea6446a9905] Merge tag 'master-2014-10-02' of git://git.kernel.org/pub/scm/linux/kernel/git/linville/wireless-next

git describe v3.17-rc7-1626-ga4b4a2b

Philippe