The digits in the Arial font supplied with Windows are monospaced, in that they each take up the same horizontal space, but they seem to have neglected to provide a "monospaced" version of the space character. This means that you can't format a column of digits right-justified in (say) 12 spaces and have the right-hand edge be aligned. For example:
1 12 123 1234 12345 1234567 12345678 123456789 1234567890
works because the font used for code examples has spaces the same width as digits. This however doesn't work if the same text is displayed in Arial (I can't demonstrate because I can't figure out how to defeat SU's reformatting at the moment!).
It just so happens that with Tahoma 8 point you can cheat because a space is exactly half the number of pixels as a digit, but that is messy and very specific.