The cancel all button is meant to be a fast way to reset your queue, however like the individual cancel button the cancel all button will not cancel any queued items that are blue (which indicates you got a skill gain from using that item).
If you use multiple crafting/gathering skills at once this may mean that clicking the cancel all button leaves an odd shaped gap in your queue where the blue items are cancelled for example…..
Queue slot 1 blue, farming skill gain 15seconds
Queue slot 2 blue, farming skill gain 30seconds
Queue slot 3 green, farming skill gain 45seconds
Queue slot 4 blue, blacksmithing skill gain 60seconds
Queue slot 5 blue, blacksmithing skill gain 1m 15seconds
Queue slot 6 green, blacksmithing skill gain 1m 30seconds
In this scenario if you click cancel all, only the green items will be removed which will mean that the next item you queue will come in at 1minute and 30seconds crafting time, and there will be a 30 second gap between the last blue farming item and the first blue blacksmithing item.
To this end, cancell all is best used when you have a queue full of one skill based action i.e. all farming resources or all blacksmithing crafted items otherwise the button will produce the effects explained above.
I did attempt to fix this problem by preventing you from using the cancel button when you had blue skill gains in the queue but evidently no one likes that solution.