Most fonts today use variable widths so that the typography looks nice, this includes the use of kerning which adjusts how certain glyph pairs are rendered when used together e.g. the letter o sitting back under the letter W in some fonts.
With variable width fonts the number 1 would obviously have a smaller width than the letter W.
By checking the Fixed width checkbox on the Metrics tab, Glyph Designer will work out what the widest Glyph is you are using and make all the glyph metrics in your font use that as their width. It will also make sure that the glyphs are centred within this new fixed width.
You'll see from the last screen show that the number 1 is now as wide as the letter W but the 1 is centred. With the letters selected Glyph Designer will show you the metrics width using the dashed line and the glyphs actual image width using the solid line.
If you want to adjust the default padding for a fixed width font you can then adjust the padding number next to the Fixed width checkbox in the Metrics tab.
You can switch back and forth between fixed width and normal within Glyph Designer as needed.