GL Impact of an Invoice shows a different AR Account than the system preference and the AR Account assigned in the Customer Record.

Settings: 
--Customer Record > Financial Tab > Default Receivables Account = AR Account 10002. 

--Setup > Accounting > Accounting Preference > Item Preferences > Items/Transactions > Default Receivables Account = AR Account 10001.

Problem: 
A User enters an Invoice for the Customer. GL Impact shows that it hits AR Account 10003. None of the above setting was followed.

Answer: 

When the Invoice was created, AR Account (found in the Invoice Form > Accounting Tab > Account Field) was changed "on the fly" before the Invoice was saved or;

Invoice was created through Transactions > Sales > Invoice Sales Orders Page and AR Account 10003 was selected in the A/R Account Field before hitting Submit.