QB Issue Resolution:

run a report on the COGS account, check each bill that shows in the listing, bills that post to COGS directly might causing the issue

You will see a bill with a small amount that might make no sense, if you sold inventory you did not have on hand, and later ordered it for stock. That small amount is the difference between the old item cost and new.

Resolution for Issue 'Total COGS on P&L is higher than total COGS on Sales by Item Summary report. What is the difference due to?' available: Yes (Solved).
Source: Intuit Community forum.

Total COGS on P&L is higher than total COGS on Sales by Item Summary report. What is the difference due to?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for COGS account
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Error Recovery for COGS account [Canada]
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