Portions of the actual picture area fall outside of the physical viewable area of our TVs, sort of like a photograph that has the matting covering the rough edges of the picture. They also have a safe action zone closer to the edge within which it is considered safe to put action that you really need to see. This does not mean we should all be happy with this state of affairs. We are losing, wasting if you will, real picture detail at the edges. Simply stated, it's our own fault.
Consumers hate seeing any fraction of an inch of their picture tube going to waste.
The funny thing is, in the computer world, we don't mind a small black border around our windows desktop at all. Go on, have a look right now. Now do the same in the horizontal. If you've managed to make either disappear off screen, you've now overscanned your monitor. I hope by now you have an understanding of what picture resolution figures represent and the potential for things to be less than they seem.