PX-755 RW writing quality

PX 755A (external), FW. 1.07
DVD-RW 4x Verbatim

DVD-RW burnt for the first time:
PIE 2,01 (av)/13 max
POE 0,50 (av)/338 (max)

same DVD-RW burnt for the 2nd time:
PIE: 46! (av)/534! (max)
POE 0,68 (av)/ 334 (max)

Why is the result so bad, when burning the same DVD-RW for the second time?
(CDRW also are much worse the second time (c1 average 10-20 instead of 2-3); same thing with plextor premium).
Are plextor burners unable to burn DVDRW/CDRW in good quality??
(just as examle: when I am buring a Verbatim CDRW with my Marantz Audio Burner the quality is constantly high, even after 20x burning the same CDRW. Thus it is not a problem of -RW media, but of the plextor burners)

Note: I split this question out of the 755-review thread, since it’s not on topic there.

As for the question:
In some cases ‘overwriting’ of RW does not work well. In such cases doing a full erase before the next burn should help.