Yes, it can fluctuate from lab to lab and the environmental conditions etc. So little bit fluctuation it can acceptable but if more then it is not acceptable.
The greater the SI value is, the more selective it is. An SI value less than 2 indicates general toxicity of the screened compound. So, it should be more than that.
In my publications I use to consider that compounds with a SI between 3 and 6 have a moderate selectivity; and those with SI higher than 6 are strongly selective compounds.