The sample consisted of the entire population.
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The sum will be zero or close to zero, depending on how the sampling was done. See related question.
There can't be a negative zero. zero is neither negative nor positive.
The scale doesn't start at zero, so you need to compromise or you get a systematic error.
This means there is absolutely no mistake in the data given.
Take a measurement with nothing in the gauge. That reading is the zero-error.