As the triple point of water is of great importance for the International Temperature Scale of 1990 (ITS-90) and for the definition of the unit of thermodynamic temperature, its long-term stability has attracted a great deal of attention. In a study of long-term stability, a mystery has been uncovered. Some triple-point-of-water cells remain stable for many decades, while others decrease with increasing age of the cells, which is called long-term drift. To investigate this mystery, we used cells with different manufacture dates ranging from 1974 to 2002 and compared their analyses, which were done in 1984 and 2003. Using the same model of long-term drift as that used by Hill, the long-term drift rates of the two data sets are –4.7 μK•year−1 and –9.2 μK•year−1, respectively. One is consistent with the observed depression of about –4 μK•year−1 measured by Hill, whereas the other differs greatly from Hill's result. In addition, corresponding factors influencing long-term drift are discussed in this paper.