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I think the point was just that you don't necessarily hear that sound in the name. You hear Will in William, but since the emphasis is on the first syllable you don't really hear Liam the same way. And you hear Rob in Robert but not Bob. The nickname makes sense but it's more than just a part cut out of the full name. Even if you pronounce William with 3 syllables, if you said Liam the way it sounds in William it would be awkward (and vice versa, it would also be awkward if you pronounced William as literally "will Liam").