DVD+RW reading speed: ISO vs. UTF


I have LG-4082B. I use Nero 6.0.3.x
I burnt two identical DVD+RW Platinum 4x disks with the same data and using the same 2.4x speed.

First one - ISO.
Second one - UDF/ISO (hybrid).

First one reads max. 2x (flat line), the other - up to 8x.
Is it normal? I don’t think so.

So, I erased (full erase) the first disk and re-recorded it using UDF/ISO file format. This time - it reads fast (up to 8x).

Did anyone have similar experiences?