Your brain can’t correct for these visual effects, so it decides on the location of the object (the straw) based solely on where the reflecting light is coming from. Above the water, the light reflects from the straw through the air and glass to your eyes. But below, when the light also travels through water, the refraction causes the image of the straw to be in a slightly different location. The water also acts as a type of magnifying lens, making the size of the straw seem larger than it actually is. To the brain, the straw appears broken (and bloated).