It is likely due to cold weather, as I frequently see the same power levels (4kW on cold days, 1.4 kW on bitterly cold days).
Also there is a service bulletin which may need to be applied to your car which can correct problems with intermittent charging behavior on some L2 chargers.
I think the charging also slows down as the battery gets full. The Clarity is 'in charge' of the charging rate.