Garlic may form "rounds" (single undivided bulbs) if grown under less than optimal conditions. That has occurred in my garden when the planting location was too wet. It could also be that you harvested the single bulb too early, before it had grown enough to divide.

The split cloves are separated because the bulb skin surrounding them has decomposed. That happens if garlic is left in the ground beyond its proper harvest date. A good rule of thumb is that each green (or partially green) leaf remaining at the time of harvest equals one layer of bulb skin.

The proper harvest time varies, depending upon the type of garlic. The tops of some "soft neck" varieties (those which do not form stems) may fall over like onions when ready. Others may be ready when a percentage of the leaves has turned brown. If left in the ground too long, the individual cloves may begin to divide... larger bulbs, but the numerous smaller cloves - each with their own skin - may be undesirable.

There are several different types of garlic which can be called "hard neck", and the indications for optimal harvest time vary. For most, no more than 1/2 of the total leaf surface should be brown. Left beyond that point, you might get slightly larger bulbs; but the bulb skins will be too thin to store well - or as in the photo, the bulb skin may disappear entirely. Open cloves should be eaten soon, or used for planting.

A good guide to proper garlic harvest times is included in the link below:

Heirloom Garlic FAQ

Thanks for the help. I'm guessing I should not leave the remaining garlic where it is, since the individual cloves will need room to grow their own bulbs, but rather dig it up and replant it at proper distance from each other (or just eat them I guess).