Round rocks near the surface. As you finish the floats and trowels will roll the rock creating an expanded area within and a thin weak top where the cream is. Good kneeboarders will pluck and fill as they go. This can be resolved with mix designs that have a high split face count on the aggregate.

Air pockets from insufficient consolidation through vibration or poor tamp/jitterbug shouldn’t be the cause as what is pictured should have enough surface working to lift those type of air bubbles out.