Different endings when typing continuous letters

[Janet Underwood]

One of my students has asked a question I've never encountered or thought 
about before. He asks why he can type the letter "d" continuously and the 
line will go all the way to the right margin, but if he types the letter "x" 
continuously on a line, it wraps to the next line before reaching the right 
margin. He points out that other letters end at different places and then 
wrap to the next line. Any ideas?

Janet Underwood