No idea, but I have another question. does any other bulb look like garlic? I pulled up a few the other day that look like garlic bulbs but I can't remember planting them where they were growing - but I DO have a terrible memory and I DO plant things in odd places.
Like you I've discovered half a dozen rock solid whitish lower half , going green at the top of the bulb . Mine definitely came from planted cloves that never quite made it into a full grown bulb .. Bad cold weather is the cause I suspect .
You can get new unintentional bulbs from seeding garlic bulbils ( they are about 1.5 mm in size ) that form in the flowers . Again I think they are only from the hardneck varieties ..
My garlic is in a permanent bed , grown to a pattern of nine cloves in marked off with twine grids of one square feet sections . So it's easy to spot weeds and things that are out of place & snip them off .
I got a bit confused when birds & squirrels decided to have a dig around in the bed and moved precisely planted bulbils & cloves .
Any doubts about a bulb being garlic cut it in half , scrape off a bit of juice from across the cut and smell it , only taste it if it smells like garlic .