Incorrect calculation led to some wrong value, I think electric charge. Subsequent findings showed a general trend towards a correct value. The general trend indicates that findings that were too far off were scared to be published for being wrong and contradicting existing studies, thus only small refinements were published, causing the slow drift.

Thus, the first well accepted study anchors a value and it takes much more work to get a new value established as correct after the anchor is set. Especially in a case where the experiment design itself is still solid and it was more with specific experiment (measurements slightly off, other values not quite correct).