Actually, if she's a noble, she could have called Raku "-sama" either way whether he's a noble or not, it's only a matter of how she thinks of him, just like you said. There are a lot of cases like that in anime, drama or movie.
In Tsugumi's case, it's more like a requirement to call Chitoge "-sama" because she is her servant, otherwise it would be very rude.
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