That's how languages are created separate from each other. In English garlic and onions are 2 very different words for closely related vegetables. I'm going to make an assumption her: Onions were known and named first to the people speaking swahili. Later on they discovered garlic and called it "a saumu kind of onions" and then late in the history of Swahili, saumu was also accepted on itself as garlic.
Vitunguu = onions(plural), right?
If so, why is vitunguu saumu not garlics (plural) but garlic (singular)?
In my use of English, British, garlic doesn't have a plural unless you refer to what we call the cloves, so we just say garlic regardless of quantity... It would be unusual to say bread and garlic cloves unless specifying the number of cloves. I've no idea about how that works in Swahili though!