TOKYO—How much is about 80? The Bank of Japan’s answers include 79, 38, and, lately, about 20.

Since October 2014, the central bank has pledged to buy about ¥80 trillion ($732 billion) in Japanese government bonds annually, a pledge it is expected to repeat at a policy meeting Wednesday. But at the current pace, the BOJ is on track to buy only about ¥20 trillion of the bonds this year, following purchases of ¥38 trillion last year.

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