Her cells, when grown in culture, have been dividing and replicating for a very long time. The fidelity of a cell's replication machinery is not 100%, such that

Her cells, when grown in culture, have been dividing and replicating for a very long time. The fidelity of a cell's replication machinery is not 100%, such that every generation, new mutations are introduced into the cell's DNA. Owing to the increased rate of cell division of immortalized cells grown in laboratory culture dishes versus those found within a living organism, the additional stresses that these cells face in an in vitro environment, and the many thousands of cell generations these cells have undergone since their derivation (in the many hundreds of laboratories that are growing some strain of these cells), the number of mutations introduced into HeLa cellular DNA makes it such that these cells are now very, very different than the original ones isolated from Henrietta Lacks.