Symfony2, LiipImagineBundle resize image exactly as width and height given in config

Question

I'm using LiipImagineBundle and I'd like to resize image exactly as width and height given in config

Right now , an original image 570px*251px is resized at 75px*33px although the size is 75px*55px in config.

liip_imagine:
resolvers:
   default:
      web_path: ~

filter_sets:
    cache: ~
    travel_75_55:
        quality: 80
        filters:
            thumbnail: { size: [75, 55], mode: inset }

LiipimagineBunde resize it sometimes 45px height, sometimes 33px height according to the height of original image I think.

How can I resize image exactly as width and height given in config ?