When selecting time range in highstock chart, y axis scaled according currently visible points and one point outside visible range. Demo: https://jsfiddle.net/85t2yjyz/
It is explicitly stated in source. https://github.com/highcharts/highcharts/blob/master/js/parts/Series.js#L684
In some cases it's not necessary, extremes must be able calculated with only visible points.
I suppose that it should be some option to control this behaviour.