why is the name William sometimes called Bill, Robert is called Bob?

Answer 1

The abbreviations probably have to do with the fact that voiced bilabial stops are the easiest form of phone for human mouths to form. A voiced bilabial stop is, for example, "b", so "Will" (which begins with a really hard voiceless bilabial glide, "w") becomes "Bill", especially to babies or those who have a hard time rectifying the tricky phonemes of English.
Likewise, "b" is used instead of "r" for "Rob" -> "Bob". It's way, way, easier to pronounced a "b" (voiced bilabial stop) than an "r", which is a voiced rhotic sound and is almost exclusive to English.
I studied linguistics for years and never, ever thought about this nickname phenomenon, how generally difficult phonemes become their simper, voiced parts upon the simplification of the name. Thanks so much for asking this question.
(Off the particulars of this question a little but still in support of its theory: "d" is also a voiced stop, though alveolar, and that's why babies can say "dada." "Mama", too, begins with a voiced bilabial nasal.)

Answer 2

http://en.wikipedia.org/wiki/William_%28name%29

This is the best I could find on connections to William, it isn't easy!

I think it starts with a name like William which is passed down through the same family, but as the name gets older, people modernize it. Maybe Grandpa's name was William, he named his son William but everyone called him "Will" to tell them apart and then "Will" had a son and decided to keep the name "William" in the family and call his son "Bill" for short.

Then I think as a name is used more and more, through different countries and time periods, it changes to something less relevant to the original name.

But who knows!?

Answer 3

Probaly parental laziness.