The solar constant isn't a constant

In a seminar at the university today, the speaker presented this figure, a graph of the solar constant (the amount of solar radiation reaching the top of the atmosphere) as a function of time from different instruments in space designed to measure it. From the web page where this figure originates: "The TSI Climate Data Record spans almost 30 years. Instrument offsets are unresolved calibration differences." I find it interesting that we don't have an exact way to measure this quantity yet.

Even accounting for instrument offsets, the solar constant isn't a constant, as it varies with the solar sunspot cycle. Furthermore, Introduction to Atmospheric Science by Wallace and Hobbs (1977) lists the solar constant at 1380 Watts per square meter, which is much higher than any of these observations.