Until there is an official announcement on this, this is my theory: The inferno and mayhem modes prompt the use of the game client's primary grimoire interface, as though a game of control, clash, skirmish, salvage, rumble, or elimination was being played. As these two game modes (inferno & mayhem) are really modifiers for actual game modes (control, doubles, rift, etc.) and the usual suspects for the game modes being modified tend to belong to the primary grimoire interface list, the game client may have a hard coded appeal to said interface during modified crucible game mode play rather than a decision fork that could handle the game client's secondary grimoire interface, which is used with unmodified doubles, combined arms, etc. game modes. An additional possibility: During inferno modified doubles weeks, the client side is actually altering your grimoire score for 25/50/100 wins in inferno doubles as though regular doubles gets this kind of scoring associated with it. If this is the case, it could explain high end grimoire score differences between players who have collected every collectible bit of grimoire available to date.