Your rank is calculated based on your total purchase amount during a specific 12-month period — from the start of the month one year and one month ago, to the end of the month prior to the current one.
For example, if your rank is shown as "Standard" on March 15, 2018, it means your total purchase amount from February 1, 2017 to January 31, 2018 was less than 50,000 JPY.
Please note that your rank is not based on the integration of all past purchases - only on purchases within the relevant 12-month period.