They also say in the article that this is not unexpected based on previous events. So I guess it is happening about every 150 years to get a meaningful average. Otherwise they are just averaging a bunch of differences in dates.
So to get an idea of how normal this is, don't we look at how many times this has happened before, and what the deviation from the 150 years is to get a better idea of what is happening?
So if this sort of melting was noticeable in 1889, and we notice it again now, then maybe we are 27 years too early.
What is the variation in the melting cycle? I can't find data.