There are something like 100 of the most common items in both CPI and RPI calculations, and whilst some might be rising in price, others might be falling or remaining the same. Inflation is a weighted average of the price rises of everything put together. It is also the weighted average for the average person. so whilst the cost of nappies or iPhones may be included, these probably aren't relevant for pensioners.
Also, rent is included in RPI and CPI.