Diaries kept by the boys' parents revealed that the negative aspects of video gaming were due to the fact that the kids with games spent a lot of time playing on them. The control group would occasionally get their hands on a joypad at a friend's house, but such opportunities only took up an average of 9 minutes a day. Instead, they spent around 32 minutes a day on after-school academic activities. By comparison, the boys who had their own games spent 40 minutes a day with them and only 18 minutes a day on after-school learning. After adjusting for these differences in work-play balance, the link between video games and reading or writing skills vanished.
This displacement explanation also explains why the boys' maths scores were unaffected - they simply don't have many maths-based leisure activities for video games to displace. Reading books is one thing but it's hard to imagine children rolling out the arithmetic worksheets for pleasure.