It is hard for governmental statistical agencies to eliminate the measurement bias that occurs whenever new and improved goods are introduced to consumers, and new and improved goods are continually being introduced. It is also hard to deal with substitution bias by updating the weights on various consumer goods.
Measurement bias is not huge around the world, and it is coming down as statistical agencies adopt new and improved statistical procedures. But there may still be some irreducible upward bias in measuring inflation. The second reason for shooting at a rate of inflation slightly above zero is known as the zero bound problem.