Yes, this is normal. In the best case quality differs from one package/spindle to the next, in the worst case the quality differs even within a package/spindle. This is IMHO one of the prime advantages of owning a Plextor drive. You can check a batch of media for yourself before putting them to use.
When a disc is produced, the dye is applied and the pregroove is stamped into the dye. The pregroove is the 'empty' spiral along which the data is burned. Now the stamper used in a production run is only good for a certain amount of discs. The quality gradually degrades the more discs are produced with the same stamper. Different grades of quality therefore emerge from a single production run and are also sold accordingly (for example the Plextor branded Taiyo Yuden +R8x DVDs are high quality from the beginning of a production run). Depending on how many discs a manufacturer wants to squeeze out of a stamper before using a (costly) replacement will determine the difference of quality you'll see from a single brand.
Unfortunately CMC seems to have a rather large span of quality in their production. They produce high quality for other companies (Verbatim, for example) but also deliver very low end resellers likewise.