Regular flour does not readily dissolve in water because it tends to lump. The outer flour molecules gelatinize soon after they come in contact with the hot water, forming a membrane-envelope that effectively blocks the water's access to the flour molecules contained within. And the result is that the inside of each lump remains dry, giving your mixture a grainy texture.
Flour manufacturers have developed instant products designed to minimize obstinate lumping and therefore dissolve more easily (though at a sacrifice to thickening and staying power). Typically, the instant product consists of small granules that have been chemically or mechanically processed into irregular shapes. When these granules do adhere to one another, spaces are left through which water can reach the middle of the lump, thus enable it to mix better.
** Asian Recipes