引起铁壳电池加底偏心的主要原因是铁壳结构缺陷和加工误差。
Major cause of deviation of bottom added from the axis of the cell with iron can were structure imperfection of iron can and processing tolerance.
答案看起来是这样的：就因为八月底是阿拉斯加北部的北极熊游向北方的海冰的特定时段，文中这只母熊也正是这样做的。
The answer seems to be that, simply, late August is the period when polar bears in northern Alaska head north on to the sea ice, and that is what she was doing.
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