Quote
It seems that these students are being literally held back from advancing in knowledge while teachers discuss broadening the scope or going deeper into the subject at the same grade level.
Yes, for some topics, a student may wish to move on to higher material. When denied the opportunity and sidelined to broader pursuits to mark time until the rest of the class catches up, these students are being held back from advancing and that is consistent with the "landings" shown in the 3-minute common core video "What Parents Should Know".

On the other hand, for some topics, some students may wish to go deeper into details which may typically be covered with students several years/grades older. (Going "deeper" at the same grade level may be seen as filler, busy work, treading water, marking time, or waiting on a landing.)

Both strategies (moving ahead and going into a topic more deeply/broadly) can be used for the time gained by curriculum compacting, a form of acceleration. Either strategy may be the right "fit" for a particular student, depending upon the circumstances. This Davidson article lists 18 types of acceleration: http://www.davidsongifted.org/db/Articles_id_10313.aspx.
Quote
8. Curriculum Compacting:
The student's instruction entails reduced amounts of introductory activities, drill, and practice. Instructional experiences may also be based on relatively fewer instructional objectives compared to the general curriculum. The time gained may be used for more advanced content instruction or to participate in enrichment activities.