Welcome to Vodafone Community
At a guess you may be on a contract with promotional extra data, if this is a good guess, you will use the core 6GB data first with the promotional extra data starting when this has been used.
The phone application will always give the data allowance at the full rate and this will include any extra promotional data.
If you need further clarification live chat or Customer Services are there to help and answer questions. Alternatively, follow this link and speak to the Social Team through Twitter or Facebook channels Contact Us