新概念第三册第三课的一个时代问题

新概念第三册第三课的一个时代问题
The body of one statue was found among remains dating from the【fifteenth】 century B.C. Its missing head happened to be among remains of the 【fifth】 century B.C. This head must have been found in Classical times and carefully preserved. It was very old and precious even then. 她的身体是公元前15世纪发现的,头颅是公元前5世纪发现,那为什么说头颅是很历史悠久的珍奇之物.我感觉既然是头颅很珍奇,它应该是公元前15世纪发现的,然后被公元前5世纪的人发现了,给它在当时做了一个身体.请解答下,谢谢了.