There are scads of contract manufacturers out there that make products for major labels to "OEM." Some of them are well-known outfits like NEC, many others are relatively anonymous offshore outfits.
Dell, for example, makes nothing. They're a marketing, support, final assembly/custom assembly, and Quality Control organization. Dell has quality control procedures like no other organization since they outsource all of their manufacturing. Dell's QC people can look over an organization from head to foot and tell them exactly what they're doing wrong, and what they need to do to qualify as a Dell supplier. If you want to do business with Dell, ISO certification is usually only a starting point on process control.
There are other sources of re-badged goods. Several name-brand manufacturers will happily produce custom productfor anyone with deep enough pockets that will commit to large quantities... so long as the sponsor's label doesn't seriously compete with the manufacturer's label.
One can make an educated guess about what the story is behind any given store label just by the product itself. Any sort of "premium" in-store brand - such as JC Penny's "Arizona" denim - are likely contract-manufcatured. Most of Target's in-store brands are either marginally re-packaged major-manufacturer goods (just look at the package - many list the manufacturer before the "Target Brands" text), or are generics either made by contract- or name-brand manufacturers for Target.