Lightroom highlight toggle not working correctly

[rgrebby]

When I hit the highlight toggle on the histogram to make all the whites show up as red it makes the bits that are not quite white are red as well.

Shouldnt it be set to 255,255,255?

 

[Chappers]

Just checked my copy of LR3 and I get the highlight alert when at least one channel is at 100%, anything below 99% is not selected.

Could it be that you have just one channel clipping, causing the area to be selected as being clipped ?