Variegation is the result of the lack of the green colored molecule of chlorophyll in a part of a plant. If you remember your basic plant science, you know that chlorophyll is where photosynthesis takes place. Parts of a plant that lack chlorophyll will thus grow slower than those with a full complement of the molecules. When on the leaf, the variegated zone is usually thinner and weaker with less substance.

Many hostas with white medial (center) variegation suffer from exposure to intense sunlight. The heat of the sun causes greater evaporation of moisture from the leaf surface. The white, thin part of the leaf is unable to keep up with the demand to replace that moisture. Thus, the cells of the leaf burst resulting in a water soaked that looks as if the leaf had "melted out".

Fortunately, this only affects a handful of cultivars that have a large area of white in the leaf so, if you don't like the effect, don't grow them.  Or, move these cultivars to parts of the garden that do not receive direct sunlight, especially in the late afternoon when it is most intense.