I am a new user and wondered why Marketo has two field data conventions. If you read this post please know that I can import - my issue is the way Marketo is set up.
Lead Database - Import CSV data field headers to not match. I matched my import data field headers to the Lead Database fields and then had to modify (a useless step) when importing a CSV file. The fied headers should be the same - cuts down on errors and cross referencing of data - not a good approach.
Examples: Lead Database data field [First, Last, Company] File import data field [First Name, Last Name, Company Name]