Hi Nicolas,
They are different on how they are connected to the measure groups. You can check the Dimension Usage tab for these information.
"Retail Channel" - This is a regular dimension directly linked by the "CHANNEL" field to multiple measure groups. For example, the "Retail Transactions Payments" measure group comes from RetailTransactionPaymentView, and you can see the field CHANNEL in the view.
"OLAP_RetailChannel" - This is a referenced dimension based on the Default dimension of the record. This dimension is linked to the measure group through the FrameworkDimensionsAttributeValueSet table.
"Ledger account OLAP_RetailChannel" - This is also a referenced dimension. However, it is based on the Ledger Dimension of the record and linked through the FrameworkDimensionAttributeValueCombination table.
In other words, a measure group might be linked to (potentially) different retail channel dimensions based on different paths. Multiple cube dimensions are created for referring to the different paths.
Hope it helps.
Regards,
Dominic