Internal Slices have the from_date_offset Attribute, which is a negative number of periods for that internal slice to go back (past) from the "From Date". For example, M_ALC internal slice has 36 periods, that should cover 12 months in the past ('-12' from_date_offset), and 24 months from (and including) the current month.
Periods for a TSV do not follow this rule, all periods are covered starting the From Date and to the future, no periods are available in the past.
Steps to Reproduce:
This is working as designed. The from_date_offset is only taken into account when calculating the From Date (e.g.: in a new Clarity PPM deployment or when doing a full re-slicing). Once the From Date is set, the periods available start in the From Date, and finish on the sum of the From Date and the Number of Periods.