You're not as alert as you think you are, even when you're trying to be. Our focus on certain things will leave gaps in other areas of our attention.
This and pax's examples are what is called inattentional blindness
. You "see" that which has your attention but are blind to that which is in your field of view due to inattention.
In short, our cerebral processors are quite limited and cannot do all things all the time. Focus on one aspect and a considerable portion of the computational power is directed at that one aspect, thereby allowing other aspects to go unnoticed.
Note that it is not just a visual phenomenon. It can be inclusive of any of the senses. Have you ever noticed somebody so focussed on an activity, say reading a hot debate on The Firingline, can miss hearing when they are called to perform some other activity or to answer a question?