I am hoping to express ACS 2013-2017 dollar-denominated data as 2010 dollars. Following this resource, my understanding is that I would take the average CPI value for 2010 and divide it by the average CPI value of 2017 (the most recent/last year in the series). This would give me the inflation adjustment ratio which I would then multiply by an ACS estimate for 2013-2017. Why is the CPI value for the last year in the series used (i.e. 2017) rather than an average CPI value for all 5-years (i.e. 2013-2017)?