It is possible that someone by mistake contributed a code that he developed for a client and therefor the copyrights for it belong to the client and not to the developer.

If I build you a house and devise new methods, custom tools, savvy workflows and creative strategies to deliver you the best house your money can buy—you don't own copyrights to my IP. You get a kick-ass final product. You can R.E. your OSS deliverables, but I'll license every-other byte. That's it. You pay, we play. You steal, no deal ;)