You created a new custom field in Salesforce and see that Marketo synced to it successfully the first time. You later decide to change the name and/or type in Salesforce. After making the change, you checked Marketo to confirm the change, but can no longer find the custom field; or, you find that the field name and type did not change.
Once a new field name and type is picked up by Marketo during the first sync, Marketo keeps the original properties. A manual change is required in Marketo in order to properly sync to the right field/type.