A lot more refresh commands are issued. A single cell holds data for 64 milliseconds. But a refresh only refreshes a subset of the RAM, a full row. So to refres

A lot more refresh commands are issued. A single cell holds data for 64 milliseconds. But a refresh only refreshes a subset of the RAM, a full row. So to refresh an entire chip you need to refresh all rows which can be hundreds to thousands of rows. So in practice you need to issue refreshes every few microseconds to ensure you hit every cell at least once every 64ms.