When your application requests a drawing surface it might not get the type of surface it requested. This means you might get a higher color depth than you requested. To avoid getting the wrong surface, check potential surfaces as they are returned and only accept the correct one.
For example, if you request a RGB565 surface you are presented with a list of configs. If you pick the first config it might be an RGBA8888 surface. This is obviously not the surface you want. If you iterate through the configs returned you can select the RGB565 format directly and avoid the incorrect formats.
For example code that shows how to sort through EGLConfigs, see the Mali Developer Center.