My perception is this,
In 11i, Subledger Accounting Feature was not made available to users/Implementers in the front end for making modifications on how accounting entry can be generated ... therefore the values entered in the distribution screen would be the same in the accounting entry level as well ... There is no chance of a mismatch between Transaction code combination and Accounting Code combination .....hence reports picking the code combination from transaction related tables or accounting related tables, does not make any difference ...
In 12, Subledger Accounting feature allows users/Implementers to make modification to the rules, thereby the accounting entry can be generated in an entirely different manner than what is entered in the Invoice distribution screen .... Hence the possibility of mismatch between Transaction Code combination and Accounting code combination exists ....hence reports picking the code combination from transaction related tables would have different values and reports picking the code combination from accounting related tables would have different values, this would eventually mislead the users .... ultimately we are concerned with the account code combination that has been used for accounting purposes, hence the reports in r12 might have been modified to pick values only from accounting related tables ..... resulting in loss of functionality ....