Most people tend to do what I call a "partial watering". Meaning you've watered the pot but you didn't get any run off from the bottom. What tends to happen here is any fertilizers that weren't used get left in the soil when the water evaporates. With organic grows this isn't so much of a problem, but with chemicals, and especially salt-based or soluble fertilizers, it leaves behind a mess.
Overtime depending on how heavy your feeding and how thorough your watering the salt that's left behind tends to build up. Think of your bath tub. You fill it up halfway for a week and you get a "ring". The same thing is going on inside your pot at the water line if you're not flushing out the old ferts. This is why most schedules say to feed every other watering, but it doesn't help unless you were watering right.
In general when you water you should make sure you're flooding pot, saturating ALL of the soil, and getting plenty of run off from the bottom. Then you wait until the pot has dried thoroughly, to the point your top begins to "droop" or fall over. This will keep you from building up excess salts which could eventually burn your plant or mess with the pH level.
