How I/O Prioritization Works (in Windows 7)

Something has confused me about I/O prioritization in Windows 7:

Normally, "prioritization" only makes a difference when multiple requests are happening for the same resource. However, 开发者_如何学GoI've noticed that when I lower my program's I/O priority, its reads always slows down the reads (e.g. from 60 MB/s to 10 MB/s), regardless of whether or not the disk is otherwise busy from other programs.

Why is this? Is I/O prioritization really I/O throttling, or does it also do something else I'm not aware of?