sb said that的意思是某人说...它所接句子的时态应该是过去时的某种时态.(客观真理除外)例：Lily said that Tom was the top student in his class.
It's said that的意思是据说.它所接句子应该根据具体情况使用相应的时态,不一定就是过去时态的一种.例：(1)It's said that he went to Paris two months ago.(2)It's said that Tom is the top student in his class.
不知道解释的清不清楚?