user vs role cache

Hello,
I have a weird problem, and pretty difficult to debug:
- I configured authcache caching for each roles
- pages are correctly cached for each user = the second time I loaded the page, I receive the cached content

... but that cache is related to the user, not to the role, if I login with a differrent user with identical roles, cache is regenerated. I would expect that all user with the same roles received the same page.
What could be the problem , authcache debug doesn't give me any usefull info ..
Is it a known prob ? Is it related to cookies ?
Any advice is welcome ,-) & Thanks,
Marc