Why sometime git-rebase doesn't preserve merge

Hello,

I'm using "git rebase -p" to rebase (not a suprise here) my devel
branch onto the master one.

In my devel branch, I've merged some remote branches.

Some merges are recreated, since I passed the -p switch,  but some are
not and all commits from the remote branch are inlined in my devel
branch.

Could anybody tell me why this happen and how I can avoid this behaviour ?

Shouldn't git-rebase fail if -p is passed ?

Thanks
-- 
Francis