Nero "Determine Max speed" is below disc rating

When I use Nero Express’s “Determine Maximum Speed” option with some 32x TDK discs on a Yamaha 3200EZ (a 24x writer), Nero gives me results somewhere in the high 20’s to low/mid 30’s. For example, I just got the following result:

Speed measurement completed: 34.6x (5,190 KB/s)

It then says:

Can only write at 16x instead of 24x, because speed of source data is too slow.

I don’t understand this. If the speed measurement resulted in 34.6x, why is it saying that the speed of the source data is too slow? The buffer never drops below 98%, so how can it be “too slow”? Wouldn’t the buffer be used up if the source data was streaming too slow?

Thanks for any information on this issue,