The title may have led you to believe that the temperature is rising worse than expected, but the comment is about the data itself.
The various sets of temperature data that we have to do climate modeling are not very good, especially as you go back in time. This shouldn't be surprising, when we're trying to detect changes in temperature on the order of a few tenths of a degree C per decade, and the state of sensors and data collection networks prior to the PC and Internet eras.
What may surprise you is how much the data we have from the past continues to be massaged and manipulated. At Climate Audit, "Worse than We Thought" outlines the recent changes to one dataset.
While the changes are suspect, the lack of transparency about exactly how the data was manipulated is near criminal.