I have had this happen too. Usually when what I think is a single clove is really 2 or 3 tiny cloves.

I agree with defigart, sometimes when separating the cloves it's easy to miss noticing that one clove is actually two or more. When my Northern White gets really big, it's common that one huge clove is actually a double thats still connected if you peel it and look closely. If this clove get planted, more than one plant develops.
I've found that if you let more than one plant develop in the same spot the garlic size at harvest is much smaller.

In my experience, this happens when the plants get too much rain too late in the season. Some varieties may be more prone to this as it seems my Korean Red does this often. Of course, it could be my fault for thinking I can get those as large as some of my other varieties.