The cells in Experiment 1 finished duplicating their DNA but those in in Experiment 2 immediately performed mitosis, with some of the cells not having replicated their DNA. That led to different amount of DNA in Experiment 2.
Work Step by Step
Look at Fig. 12.14. You can tell from the amount and thickness of the blue lines where cells differ in the amount of DNA. To see the problem for G1 cells, review the chapter material on what has happened at different stages of the cell cycle.