Thats typical of anything new, not just rebuilt engines. Initial usage exposes flaws in components that exist from manufacturing or assembly errors. After that you typically get a troublefree time period and then finally you reach the timeperiod of failures due to wear and simply the life cycle of the components. Its a "U" shaped failure rate. Junkyard engines are almost always much closer to the right side of the "U". The last owner probably knew that and probably treated them accordingly too.