Suppose you're interested in global warming, just in understanding what's going on -- not in 'proving' that there is warming, or cooling, or that temperatures are unchanged. You're an actual skeptic -- looking for evidence and where the evidence leads. One thing you learn pretty quickly in your skeptical explorations is that you need 20-30 years of data to define a global climate temperature. Shorter than that, and your answer depends sensitively on your averaging period. As a skeptic, you don't want such unreliable methods. Apply the 30 years to a number of data records (below), and you get the answer that climate has been warming, 1.3-1.7 K/century (2.3-3.1 F).
As a cherry-picker, committed to finding a particular answer, however, you go straight for the option of using short spans -- look for a record length that will give you the answer you want, then ignore the fact that your answer changes if a couple years are added or subtracted.