Since I started reading about the stories of the lives of young English youths written by Enid Blyton, I’ve always wondered about this. Why is Bob the short form of Robert… and Bill for William, and Daisy for Margaret and… all the other short form names that doesn’t sound at all like their extended version… wonder if anyone can help to shed some light on this?
I’ve never heard of the Margaret/Daisy thing, Margaret is most often shortened to Maggie or Meg. I did find this however.
Robert-Robby-Bobby-Bob-Rob-Rab seems to me like a fairly logical continuum of variations. Likewise William-Willy-Billy-Bill-Will.
I’d never heard of the Daisy/Margaret one, either.
It’s most likely Margaret was given the nickname “Daisy,” much like George Herman Ruth was named “Babe.” One wouldn’t draw the conclusion that “Babe” is a common nickname for “George.”
The origins of common nicknames is not always obvious. “Peggy” for “Margaret,” for instance. Even the use of “Chuck” for “Charles” is somewhat arbitrary – “chuck” was a general term of endearment that somehow attached itself to “Charles.”
As someone posted above from the SD article, Daisy is a nickname for Margaret from the French. Also, in a time when there were less name available and you had a bunch of, for example, Roberts, you needed a way to distinguish them. It wasn’t a bit uncommon to just change one letter or one vowel to get a nickname. So you’d have Robert, then Rob/Robbie. So now we rhyme it to Bob/Bobby, Dob/Dobby, and Hob/Hobby (a couple of forms we don’t hear often anymore). You also had the diminutive Robin, as well as nicknames like Dobbin.
Same thing with Will/Bill, Rick/Dick, Meg/Peg. You’ve got vowel shifts too, like getting Meg from Mag (from Margaret of course). Already have a Rob and Bobby and Dobbin, so need something new? Shift Rob to Rab. A lot of Roberts were called Rab. Sometimes it’s even internal letters that get switched, like going from Mary to Maly/Mally to Molly to Polly. Diminutive endings get us not only the aforementioned Robin, but also Megan from Margaret.
There’s a progression from John to Jack as well, using some of these same concepts, although it spans some cultures. I’m not sure if I can remember it off the top of my head though. It’s something like John, Jonkin, Jankin, Jakin, Jack, but I feel like I’m missing something or have something wrong in there.
It’s all pretty fascinating. I spent part of a summer one time trying to trace all the forms of Mary, Margaret, Elizabeth and John. This included foreign variations as well as nicknames and diminutives. It was pretty cool. Mary by far has the most, with John second.
Daisy historically (until this century) has been a common diminutive of Margaret though the alleged Cecil disagrees about the contributor’s assertion.
One of the most common annual daisies is called a marguerite as well.
Some of these nickname variants apparently come from the historical fondness of English speakers for wordplay, which morphed Rob(ert) to Bob, Meg (Margaret) to Peg, and Will(iam) to Bill as rhyming versions of the original nicknames.
The guy you find floating in the river…no arm… no legs…
Dat be BOB. 
Another royal connection for the Margaret / Daisy link:
HRH Princess Margaret of Connaught (1882-1920), grand-daughter of Queen Victoria, was nicknamed “Daisy”.
…and, if you take the same guy, and toss him in front of an entry door, his name changes to MAT.
don’tgetmestarted…
Marguerite is french for daisy.
You also sometimes see “Nan” for “Anne”. (Anne Boleyn was nicknamed “Black Nan.”) I’ve even seen “Nancy” used as a nickname for Anne.
Further back from that, Princess Marguerite of France (1282-1317), the second wife of King Edward I of England, was known as Daisy.
It’s baby talk, I keep telling ya!