Because a common characteristic of gifted learners is that they may require fewer repetitions to grasp a concept, avoiding or minimizing repetition means that material will be covered more quickly.
For some topics, some students may wish to go deeper into details which may typically be covered with students several years/grades older. For other topics, some students may wish to quickly move on. Both of these may be considered forms of acceleration. This Davidson article lists 18 types of acceleration:
http://www.davidsongifted.org/db/Articles_id_10313.aspx. Limiting students to classes which are one year accelerated may be better than nothing, but as others have mentioned, may fall short... far short... of serving the gifted population.
