This is because if the end of the tube below the water level it will slowly siphon water from the tank, lowering the tank level.  At some point the fill valve will open to bring the tank back up to the full level.  This process will cycle continuously, wasting water and getting on your nerves.

It's not obvious why/how this happens.

The diagram below is an incomplete attempt to illustrate the mechanism.  I adapted this figure from a Fluidmaster fill valve illustration at the bottom of the page, but the principle is the same for most if not all fill valves.  My illustration is not entirely complete, but I couldn't find an adequate diagram of the inside of an entire fill valve assembly.  The fill valve assembly has an inner pipe/tube carrying the fill water from the supply line, and an outer shell/tube almost full of tank water.  The refill tube siphons tank water from this outer shell.  This is just one fill valve design.  There are many others but they all work basically the same way.  They all have refill tubes and will siphon through the refill  tube in this same way.

You may want to review how the fill valve itself works at the bottom of this page before trying to understand how the refill tube siphon action works.  A key point is the partial vacuum in the fill valve draws tank water above the level in the tank itself, raising it to the refill port.

The following explains the operation of the Fluidmaster 400 series fill valve (ballcock)