Earlier quoted context omitted.
But it isn't "Bob" to "Robert". You skipped middle step, and one final step. First names were shortened. Robert to Rob Richard to Rich William to Will Then the shortened version had first letter replaced to make second diminutive form Rob to Bob Rich to Dick Will to Bill Finally you have third diminutive form, by adding ending Bob to Bobby Dick to Dicky Bill to Billy
Richard to Ric to Dick is mentioned here: http://www.todayifoundout.com/index.php/2012/04/how-dick-cam... Also says that dick didn't mean cock until a lot later.
Probably nothing, but still pretty awkward as a foreigner. You'd think these names would fall out of style but apparently everybody tries their best to be mature about it.