Grunt imagemin - 'optimizationLevel' not working

Question

Everything is fine except one case is that optimizationLevel is not working.

I dont get result based on optimizationLevel level change (level from 1 to 7)

imagemin:{
        dynamic:{
            options:{
                optimizationLevel: 1
            },
            files:[{
                expand: true,
                cwd: 'work/imgs/',
                src: ['*.{png,jpg,gif}'],
                dest: 'work/imgs'
            }]
        }
    }