This should be obvious enough to not need pointing out, but at the time of the Bengal famine there was a world war in which Japan was invading Asia and Germany was invading Europe. Many of the things that exacerbated the Bengal famine were due to defensive measures designed to avoid even worse deaths if those invasions were successful.

In contrast, the famines in the USSR and PRC weren't caused by invasion. They were caused by the collectivization of agriculture for ideological reasons.