All fonts, except the italic, share the same figures and monetary symbol metrics. In other words, those characters are monospaced and have a width of 7 pixels. This allows you to easily design statistical, tabular data or any other numeric information - the line width always stays the same. For the same reason the space character has the same width in every font. Italic weight is the exception to this rule, because it's so narrow (and rarely used for stats and tables).