Sorry for not answering earlier, I didn't notice this forum had any new posts.
The % complete for a CD-RW erase comes from the sense area data returned by the drive. If it's wrong, it's a bug in the drive's firmware.
I don't know what the deal is with the DVD+RW, maybe the drive knows that the disc has already been formatted and is just finishing really quickly.
The program submits the format command to the drive, the drive then goes off and does the foreground part of the DVD+RW format. Once it says that's complete, and assuming ImgBurn is set to format discs properly (which yours is), it queries the drive on the status of the background format process. If the drive is giving the program the correct responses, it should wait until that's totally finished... however long that takes... and display % complete as reported by the drive.
I don't have a DH20A4P drive and I can't locate my DH20A4H, so I just ran a test on a DH20A1S... that one seems to full erase a DVD+RW (MKM-A02-00) how I'd expect it to.
I 21:18:18 ImgBurn Version 184.108.40.206 Beta started!
I 21:18:18 Microsoft Windows 7 Ultimate x64 Edition (6.1, Build 7601 : Service Pack 1)
I 21:18:18 Total Physical Memory: 16,760,108 KB - Available: 9,663,640 KB
I 21:18:18 Initialising SPTI...
I 21:18:18 Searching for SCSI / ATAPI devices...
I 21:18:18 -> Drive 1 - Info: ATAPI DVD A DH20A1S UX14 (G:) (ATAPI)
I 21:18:19 -> Drive 2 - Info: ATAPI iHAS624 B GL2A (M:) (ATAPI)
I 21:18:19 -> Drive 3 - Info: HL-DT-ST BD-RE BH12LS38 1.01-D0 (N:) (ATAPI)
I 21:18:19 -> Drive 4 - Info: Optiarc DVD RW AD-5280S 1.Z7 (P:) (ATAPI)
I 21:18:19 -> Drive 5 - Info: PIONEER BD-RW BDR-207M 1.21 (O:) (ATAPI)
I 21:18:19 -> Drive 6 - Info: DTSOFT BDROM 1.05 (Y:) (SCSI)
I 21:18:19 -> Drive 7 - Info: DVDFab Virtual Drive 1.1 (W:) (SCSI)
I 21:18:19 -> Drive 8 - Info: ELBY CLONEDRIVE 1.4 (V:) (SCSI)
I 21:18:19 -> Drive 9 - Info: VXDV BD-HD-DVDRAM S15 10.0 (X:) (SCSI)
I 21:18:19 Found 1 DVDÂ±RW, 2 DVDÂ±RW/RAMs, 1 BD-ROM, 2 BD-ROM/HD DVD-ROMs, 2 BD-REs and 1 BD-RE XL!
I 21:18:51 Operation Started!
I 21:18:51 Device: [0:4:0] ATAPI DVD A DH20A1S UX14 (G:) (ATAPI)
I 21:18:51 Media Type: DVD+RW (Disc ID: MKM-A02-00)
I 21:18:51 Media Supported Write Speeds: 2.4x, 4x
I 21:18:51 Quick Erase: No
I 21:18:51 Format Properly: Yes
I 21:18:51 Format Size: Preferred
I 21:18:51 Erasing Disc...
I 21:34:14 Operation Successfully Completed! - Duration: 00:15:22
As for the full erase on a CD-RW, I can confirm the DH20A1S has the same bug in its firmware that your drive has. The progress bytes in the sense area data returned by the drive are set to '08 46' (which translates to 3%) for the entire erase.
At least they've fixed it in their newer models - my iHAS624 is fine.