A comparator when placed facing a container with a block in between detects it fine until a redstone signal of 15 is applied to that block, overriding the containers value to the comparator. This doesn't occur when a signal strength of 14 or lower is applied.

In the video both droppers contain 26 items each, causing the comparators to output a signal strength of 1. When a redstone signal is applied to the blocks, if the strength is 15 the comparator changes to output a strength of 15 while the other comparator doesn't change, When this redstone line is turned off it resets to default.