Mapping not working?

Apologies if I've miunderstood.

Given a CSV file like this:

Given Name,Fave colour
Rich,cyan

And a getMapping() that returns

[
   'first_name' => [
      'label' => 'First Name',
      'field' => 'first_name',
      'aliases' => ['given name'],
    ],
    'colour' => [
       'label' => 'Favourite Colour',
       'field' => 'colour',
    ]
]

I thought I'd be dealing with rows including first_name and colour? But I get given_name and fave_colour?

Which suggests the mapping is not being applied?