Hmm that's weird, seems like your guy doesn't like playing more than 61 minutes or else he drops in DMI :)
Obviously there isn't enough date here to prove that but it can be interesting, maybe you need to get 48-60 minutes for this particular guy to increase in DMI and GS.
This can be a very interesting study, to prove how many minutes = a pop in GS for someone with "so and so" ST or with a certain age or something.