Hansen's data set at NASA GISS is used by all of the other surface temperature labs in the world, and it represents most of the surface temperature station measurements available.
Over the years Steve McIntyre has been able to show how the folks at NASA GISS have been altering the temperature data set. The effect of this is to make more recent times appear to be warmer. They apply "corrections" of older data which makes the weather in the past look cooler. They do it to one grid at a time on an ad hoc basis, so that much of the older record has been altered. The "corrections" of older temps are always in the same direction, downward, which makes the recent temps look higher. They will not make the protocol used to make these corrections public or even explain why they are done.
In addition, up to 30% of the temperature records are simply made up to fill gaps in the raw data. This is done using some sort of interpolation protocol the nature of which they will not make public.
No, there is no concievable legitimate reason to alter the older data once it has been collected and entered in the data set. In any case, the effect of the successive "corrections", made over the years, is to make it possible for Hansen to make the claim that the climate is warmer now than ever before.
Previously, Steve McIntyre was able to point out an error in the GISS data set that, when corrected, made 1937 the hottest year on record. But more of Hansen's corrections were made, and 1937 subsequently fell to second place.
The CRU has done the same thing in successive IPCC reports. They referred to this in their emails as getting rid of the 1940's blip.
Anyone who has followed Hansen's public pronouncements on this issue, such as his recent condemnation of Copenhagen as not being enough to correct global warming, such as his call for riots in the streets over global warming, would not doubt that he is capable of falsifying the data in support of his political agenda.