It would be good if you can post a log from ImgBurn of the burn in question, if you go into Help on ImgBurn toolbar you will see that you can access past logs from there. Just copy and paste the one you want into your reply and we will take a look.
If I had to guess I would say that all is ok, none of my drives start burning at 8x, 12x or 16x they all start out slow and build up to speed during the course of the burn - I would bet that this applies to everyone here on the forum. As for Nero - it lies.... I have to use it at work sometimes and it claims to be burning at a certain speed but if you take the time to work it out you can see that it's not.
How long did Nero take to burn the same dvd that ImgBurn did ? If you compare burn times I will be surprised if they are not almost the same, which means that Nero just reports the speed you requested instead of the actual speed like ImgBurn.