Harvey: I don't know if it works the same on everybody's computer, but here's how it works on mine: When you type your message into the message box, it appears in Courier font, but after you press "Submit Message" and the message is added to the thread, your browser shows it to you in Times New Roman font. Courier is a fixed-width font; all characters, including the space, are the same width. Times New Roman is a variable-width font; some letters are wider than others, and spaces are very narrow. Compare these lines:

In Times New Roman:
iiiiiiiiii
rrrrrrrrrr
ssssssssss
aaaaaaaaaa
bbbbbbbbbb
mmmmmmmmmm
Note: a space is slightly narrower than the letter "i".

Here's the same text in Courier:
  iiiiiiiiii
  rrrrrrrrrr
  ssssssssss 
  aaaaaaaaaa
  bbbbbbbbbb
  mmmmmmmmmm

Moral: For predictable spacing, force everybody's browser to display the text in Courier. You do this by using HTML "preformat" commands; i.e. by inserting "<pre>" before and ""</pre>" after the text that you want to show in Courier.