What is this joke about?

In ancient times, years were counted by dry branches. The line is heaven, that is, A, B, C, D, E, G, Xin, Ren and Ghost. Branches are earthly branches, namely, Zi, Ugly, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu and Hai. Some people like to tell jokes with heavenly stems and earthly branches's homonym, and Wang Wanxu in Ming Dynasty is one of them.

Wang Wanxu was a scholar of Wanli Chenjia (1604), and was originally a magistrate of Zouping County, Shandong Province. One day, he met the county magistrate of Zhangqiu, a neighboring county. Zhangqiu county magistrate asked him when he was born, and replied, "Yihai (1575)." He asked the other person again and replied, "It's easy sea, too." It turns out that they are the same age. Wang Wanxu jokingly said, "I am Zou Pingyi (Yihai) and my brother is Zhang (Yihai)!"