Part of the difference is attributed to the fact they were a part of different Rates. The Librarian of the Lifeworkers, and the Didact of Warrior-Servants. Each rate goes through rites of passages known as mutations that alter their appearance and chemistry/biology. Forerunners can range from having 4 to 7 fingers if I recall correctly. Of course, the Didact didn't always look so... extreme. For the majority of his life, he looked like [url=http://www.halopedia.org/images/2/22/H4-Terminal-Didact.jpg]this.[/url] That changed when his mind was molested by a Gravemind. The psychological experience was so traumatizing, it had a physical effect on his body.