The formula for the harmonic mean is
2/mean(1/var1, 1/var2))
I would not expect the absolute value of the mean (whatever mean) of two values to be larger then the absolute value of each of the values. However, for the values $-800.2756851$ and $883.1436892$ this is the case with $-17057.45$. What is going on? What would be a robust version of the harmonic mean that does not have this problem?