The Torah (Leviticus 23:32) says regarding Yom Hakippurim וְעִנִּיתֶם אֶת נַפְשֹׁתֵיכֶם "and you shall afflict your souls."
The Mishna (Yoma 8:1) gives the prohibitions of Yom Hakippurim as follows:
יום הכיפורים אסור באכילה ובשתיה וברחיצה ובסיכה ובנעילת הסנדל ובתשמיש המיטה.
Yom Hakippurim is forbidden in eating, drinking, washing, anointing, wearing shoes, and having relations.
The Torah (Numbers 30:14) also uses the same phrase, לְעַנֹּת נָפֶשׁ "to afflict the soul," about annulling vows, and the Mishna (Nedarim 11:1) interprets:
ואלו נדרים שהוא מפר דברים שיש בהם ענוי נפש, אם ארחץ ואם לא ארחץ, אם אתקשט ואם לא אתקשט.
These are the vows he can annul, things which have in them affliction of the soul: To bathe or not to bathe, to wear jewelry or not to wear jewelry.
Or according to Rabbi Yosi (Nedarim 11:2):
ואלו הם נדרי ענוי נפש, אמרה קונם פרות העולם עלי, הרי זה יכול להפר.
These are vows of affliction of the soul: If she said, "All the fruit in the world is forbidden to me," he can annul.
In both cases, the Torah uses the same words (עינוי נפש, "affliction of the soul"), and yet the Mishna interprets both of them independently and differently from each other.
Why is the same phrase interpreted differently in the context of Yom Hakippurim than in the context of vows?