Since Windows XP or before, you are able to force delete a file by holding down shift while pressing delete.
For some time now I've noticed if you select a LARGE number of files (e.g. 20k) or so, shift-delete does not force delete - instead it presents the option to send to recycle bin in the same way as if you had just pressed delete.
In trying to consistently reproduce this error I have found that it is not based on a pure threshold. Sometimes I am able to properly shift delete 10k files, sometimes it prompts me to recycle 5k.
It makes me suspect that something asynchronous is going on in the background but I can't really speculate as to what.
The short and sweet of it is, is there a way I can consistently shift-delete a large number of files?
Running W7 64-bit for what it's worth. I think i experienced this in XP as well but can't say for certain.