This looks like it might be a bug in the stock GUI slider implementation, as I'm not even receiving input events for many of the numbers that get skipped (-0.3, -0.4, -0.5, -0.8, -1.0)

Will continue investigating for a bit, but this might have to wait for 1.1 and the custom GUI controls I have planned.

Indeed, the widget is not even switching to to the skipped values; something in the back-end math for calculating intervals is... not working properly for negative numbers. And as it is a stock piece of code, there is nothing that I can do to fix the current implementation.

Will investigate this one again during the 1.1 update -- as I'm planning on making a custom GUI widget for that purpose anyhow, fixing this problem should come as part of the proper implementation of the widget.