I won't go back into all the details, but I have posted before about just how large the manual adjustments to temperature numbers are (the "noise") as compared to the magnitude of measured warming (the "signal"). This issue of manual temperature corrections is the real reason the NASA temperature restatements are important (not the absolute value of the restatement).
Here is a quick visual example. Both charts below are from James Hansen and the GISS and are for the US only. Both use basically the same temperature measurement network (the USHCN). The one on the left was Hansen's version of US temperatures in 1999. The one on the right he published in 2001.
The picture at the right is substantially different than the one on the left. Just look at 1932 and 1998. Between the first and second chart, none of the underlying temperature measurements changed. What changed were the adjustments to the underlying measurements applied by the NOAA and by the GISS. For some reason, temperatures after 1980 have been raised and temperatures in the middle of the century were lowered.
For scientists to apply a negative temperature adjustment to measurements, as they did for the early 1930's, it means they think there was some warming bias in 1932 that does not exist today. When scientists raise current temperatures, they are saying there is some kind of cooling bias that exists today that did not exist in the 1930's. Both of these adjustments are basically implying the same thing: That temperature measurement was more biased upwards, say by asphalt and urbanization and poor sitings, in 1932 than they are today. Does this make any freaking sense at all?
Of course, there may be some other bias at work here that I don't know about. But I and everyone else in the world are forced to guess because the NOAA and the GISS insist on keeping their adjustment software and details a secret, and continue to resist outside review.