Although I have had some historians disagree, there is evidence that the 1st Maryland CSA (re-designated 2d MD on the monument) fought directly against fellow Marylanders on Culp's Hill. Harry Pfanz relates the first hand account of Pvt. D. Ridgely Howard, company A, 1st Maryland CSA, who was wounded in the attack on July 3 and lay on the field expecting to die. Two Union soldiers dragged Howard into the Union lines. They asked Howard if he was aware that he was fighting fellow Marylanders. Howard answered, "Yes, and we intend to fight them." Pfanz also quotes from a letter by Col. James Wallace, commander of the 1st Maryland, Eastern Shore (Union) to Bachelder. Wallace spoke of the aftermath of the battle when he says his men "sorrowfully gathered up many of our old friends and acquaintances [from the Confederate battalion] & had them carefully & tenderly cared for." Perhaps some were even laid in the gravesite you mention.
One can only try in vain to imagine the emotions of gathering up dead and wounded enemy soldiers on a battlefield, and discovering that the very men you were shooting at and trying to kill or main had once been your friends.