The phase-change material used in making DVD+RW (or CD-RW) has a somewhat predictable progression in burn quality with use:
1st burn on a virgin disc always yields the lowest error rates.
2nd burn (1st rewrite) causes a dramatic jump in error rates.
Error rates will ease back down by the 5th to 10th rewrite, but never back to as low as the 1st burn.
Error rates will climb again as the rewrite count increases, mainly due to aging.
Since the full capacity is rarely utilized during a write/rewrite, different areas of the disc can have different write/rewrite counts as the disc gets used, thus such a disc can show several levels of error rates within a single scan. Don't be surprised if the front part of the disc gets higher error rates, as inevitably the front part gets rewritten more frequently.
Also, it is very rare for rewritable media to have error rates as low as write-once media. This is true for both DVD and CD writable discs, and is something we have to live with. The higher error rates and lower reflectivity of DVD+RW is why most drives read this media slower than DVD+R/DVD-R.