I've noticed the same on my Venture Cx. It counts in minutes until it gets to 120 minutes, then changes to hours and minutes.
I'm pretty sure it used to change to hours at 60 minutes - I think a recent software update changed it. I don't know whether this was an intentional change by Garmin, or if there's a setting anywhere to change this.
Though its not really a problem IMO, just a bit strange. And I've mostly got used to it now.