halacha - What was the ריב"א thinking?

In ריקאנטי number 166 (brought down here, #4), he brings a story about the ריב"א:

ושמעתי כי ריב"א חלה חוליו שמת בו וחל יוה"כ באותם הימים וא"ל הרופאים אם לא תאכלו ודאי תמות ואם תאכל שמא לא תחיה והוא ז"ל אמר ברי ושמא ברי עדיף ולא רצה לאכול ומת תנצב"ה

I heard that the ריב"א became sick with the illness that he would eventually die from. When Yom Kippur came, the doctors told him "If you don't eat, you will certainly die; if you eat, maybe you'll live." He said "ברי ושמא ברי עדיף (a certain thing and an uncertain thing, the certain thing takes precedence -- a Talmudic principle)," he didn't want to eat, and he died.

The ריב"א is dead wrong, at face value -- of course we not only allow but require people to eat on Yom Kippur when necessary in order to preserve their lives, even when it's only a doubtful case of פיקוח נפש. (See, e.g. Shulchan Aruch, Orach Chaim 328; Rambam, Hilchot Shabbos 2)

What might the ריב"א have been thinking?
Also, what is the "ברי ושמא ברי עדיף," what is the "ברי," and what is the "שמא"? The certainty that he would die? The certainty that he gets a mitzvah for fasting on Yom Kippur, and the uncertainty of his survival? His reasoning doesn't appear to make much sense.

Answer

Shaalos Uteshuvos Radbaz 3:444 says that perhaps the Riva felt that his time was up and knew that he would die either way and therefore did not eat.

"אפשר דריב"א ז"ל הרגיש בעצמו דאפילו שיאכל ימות ד'לב יודע מרת נפשו', והיינו דקאמר: ברי ושמא ברי עדיף. כלומר: הברי שלי [שאמות] עדיף משמא שלכם [שאולי תצילו אותי], ולפיכך לא רצה לאכול".‏

The "ברי" was the fact that he felt he would certainly die, whether he ate or not; against the שמא that the doctors thought that eating might heal him.

Thanks to http://www.daat.ac.il/mishpat-ivri/skirot/111-2.htm for this answer.