Oil supply and demand don’t fully justify the 60 percent drop in prices between June and January as speculation also played a role, according to OPEC. A rise in supply from outside the Organization of Petroleum Exporting Countries at a time when demand for oil was weak was the main reason for the drop, OPEC said in its monthly bulletin on Monday. Brent oil, benchmark for more than half of the world’s crude, tumbled from mid-June to a six-year low in January as U.S. output climbed to the highest level in more than four decades and OPEC members pumped more barrels.