That light in those winter afternoons isn't uplifting like the light of summer. Nope. No sunscreen needed here. Rather, it's isolating in "There's a certain Slant of light." The speaker has her feelings put on the spot because of that light, making everything around her seem oppressive like that "imperial affliction."
They may seem like close cousins, but isolation and sadness are two different things in Dickinson's poem. Isolation indicates the speaker's "internal difference."
Not so fast there. Isolation leads to sadness, so the two are inseparable in Dickinson's poem and help to create the speaker's feeling of an "imperial affliction."