This is quite an illogical/bizarre argument you're making here. Put simply, reducing benefits for future claimants is, and always will be, a drop in those benefits.
During the leaders debate Cameron very very clearly, without caveat, stated child tax credits would not fall. This would not have been the case had the Lords not intervened.
Spring Hall Convert brought it up very clearly in the context of, and in comparison to, the current alleged constitutional crisis. Nothing to be curious about.