It's simply that, if you look at the data, the bias in ship measurements happens to go the other way. (variable-variability.blogspot.ru)
But in this case, a given measurement may be unique in space — ship measurements for example. (judithcurry.com)
Due to the limited availability of ship measurements, the measurement data are much more «blurred» than the high - resolution model data. (realclimate.org)