The variation used on the frontend are not dependant on the siteaccess currently used, instead the default one is used.

Steps to reproduce:
1. Define two different siteacceses in ezplatform.yml

    siteaccess:
        list: [site, second_site]
        groups:
            site_group: [site, second_site]

2. Define one image variation for each site access, with the same name but different parameters:

    system:
        site:
            image_variations:
                my_var:
                    reference: null
                    filters:
                      - { name: geometry/scaledownonly, params: [500, 500] }
        second_site:
            image_variations:
                my_var:
                    reference: null
                    filters:
                      - { name: geometry/scaledownonly, params: [250, 250] }

3. Login as admin to the admin panel
4. Create a new Image in Media/Images
5. Start creating a new Article
6. In the "Body" field embed the Image created in Step 4
7. Select "my_var" as image variation
8. Publish
9. Visit the created article on the frontend in both siteacceses.

Expected:
Images is displayed in different size in both siteaccess, as the image variations have different sizes specified.

Actual:
Images are displayed in same size (of site siteaccess definition).

Note:
I've added logger to the AliasGeneratorDecorator class and verified the cache key used when accessing the second siteaccess:

[2018-08-16 10:32:55] app.DEBUG: Cache key:    ez-image-variation-second_site-http-ezp.local-80-53-508-187-my_var [] []
[2018-08-16 10:32:55] app.DEBUG: 'my_var' variation on 7/8/1/0/187-1-eng-GB/Screen Shot 2018-08-13 at 11.17.34.png is already generated. Loading from cache. [] []

But the directory web/var/second_site does not exist (web/var/site does).