In creative mode it is possible to give yourself stacks of items (even if they do not stack). But these stacks can also be placed inside a block which have an inventory, for example a hopper, which does affect the "internal" signal strength of a comparator.

What I expected to happen was that the internal signal strength of a comparator has an maximum of 15 (just like redstone wire).

What actually happened was that it can be bigger (64*15=960 as max due to placing stacks of 64 of not stackable items in each slot). And the comparators can sort eof calculate with these values as well as long as the signal isn't passed along by redstone wire (so one block between two comparators or a comparator directly pointing into the back of the second comparator)

Edit: I can't seem to attach screenshots, so I made a Imgur album: http://imgur.com/a/jqyvf