APT Chart, page-1580

Here's the maths for you (using ANZ, which is their only relatively mature market):

FY17 and before cohort (840k customers, 23 transactions per year) - FY20 GMV of $3.0b
FY18 cohort (1360k customers, 14 transaction per year) - FY20 GMV of $2.9b
FY19 cohort (610k customers, 7 transactions per year) - FY20 GMV of $0.6b
FY20 cohort (490k customers, 6 transactions per year) - FY20 GMV of $0.4b
Gives a total of 3.3m customers generating $6.9b of theoretical GMV. It looks like FY20 actual GMV for ANZ will be $6.3b.

$6.9b is not multiples of $6.3b (ie the numbers do line up quite closely), so clearly your analysis is flawed.