Sometimes it is possible that Gross/Net Profit shown in Profit and Loss Statement report is not matching with Gross Profit in Sales Register. There can be several reasons for this:
Sales Register shows you the details of Invoices. It displays Gross Profit in a new column as below:
Gross Profit of an invoice = Sale Price of items - Cost of items*
* This cost of your Inventory Items is computed on the basis of Purchase vouchers entered for that Item or the Initial cost specified for Opening Stock & the valuation method selected (AVCO or FIFO)
Gross Profit in Sales Register considers only the cost of Inventory Item while computing profit and does not take any other business expenses incurred.
Profit and Loss Statement
Here Gross Profit = Total Sales - COGS* - Direct Expenses + Direct Income + Debit Note - Credit Note
* COGS = Opening Inventory + Purchases - Closing Inventory
Net Profit = Gross Profit - Indirect Expenses + Indirect Income
In Profit and Loss statement, various Direct/Indirect Expenses/Incomes are considered before arriving at Gross/Net Profit figure. But this is not the case in Sales Register. Therefore it is likely that the two reports show you different values for Gross Profit.
Similarly, inventory item details reports shows you the profit for every item. So, it measures profit from different angle.
In short, all 3 reports may show you different profits and their use is to find out profit from every aspect.