Ernest
Kriging can produce negative values from an all positive input dataset.
I used your data and repeated the kriging 100 times without changing any of the parameters. All the outputs are identical, which is what we would expect.
By the way, you have coincident data. The kriging method in Spatial Analyst will only use one of the coincident values, whereas, kriging in Geostatistical Analyst can use all the coincident data.
Steve