I found this reply in freeradius mailing list in 2005:

" It's impossible to enforce traffic limiting *during* a users
session.  So if a user is a tiny bit below their limit and logs in
again, they can go over their limit.  The server will only catch &
enforce their limit on the next login.
  This has been discussed multiple times on the list over the past 5
years."

Is this possible now in new versions ?