Why does the HSN summary not match the Sales By Item report?

The value in HSN summary is different from Sales By Item report due to below reasons:

• In the Sales By Item report, the data is not based on the tax association of transactions (whether tax is applied to a transaction or not). However, the data shown in the HSN summary includes only taxable transactions.
• In the Sales By Item report, only items created from the Items module are reflected. If you have associated any Service Item (typed as a description) while creating an invoice, those will not be displayed. However, in the HSN summary, all items will be displayed.