There is always some reason why columns never converge in time.

Johnson's division, which was on the extreme left of the rebel army, and had not been engaged, made their way, sheltered by the ravine of Rock Creek, to assail the right at Culp's Hill, held by Wadsworth's division of the First Corps, and that part of the line still farther to the right where Geary's division of the Twelfth Corps was posted.
In his desire to reinforce the Fifth Corps at the close of the conflict with Longstreet, General Meade made the sad mistake of ordering the Twelfth Corps to abandon its position on the right and report to General Sykes for duty on the left.

General Slocum, sensible that this would be a suicidal movement, reported that the enemy were advancing on his front, and begged permission to keep Geary's division there to defend the position.

General Meade finally allowed him to retain Greene's brigade, and no more, and thus it happened that Ewell's troops, finding the works on the extreme right of our line defenceless, had nothing to do but walk in and occupy them.

If Meade was determined to detach this large force, there seems no good reason why two of Sedgwick's brigades should not have been sent to take its place, but nothing was done.
Johnson's division, as it came on, deployed and crossed Rock Creek about half and hour before sunset.