The really simple explanation for this becomes apparent when i compare the Trade data to the Transaction Tax data. As a reminder, every sale that is made on the market attracts a Transaction Tax of anywhere between 1.0% (Accounting Skill Level V) to 2.0% (no Accounting Skill).
Now, i would expect the Transaction Tax divided by the Trade data to be 1.0% because the vast majority of all Trade will be done by characters with Level V Accounting.
And indeed in February, March and April the Transaction Tax divided by the Trade data was 1.00%.
But then along came Citadels and the Transaction Tax divided by the Trade data rose at first to 1.31% and in November is now 1.69%.
That suggests to me that either the Transaction Tax data is too high or the Trade data is too low.
If i assumed that the Trade data was too low and if i assumed that the Transaction Tax divided by the Trade data was still 1.00% then i can deduce what i think the Trade data should be - see the table below: