It is pretty easy, you play against people who have more or less points then you. If they have enough 'less' points that they are into a lower zone they are allowed to use a different set of cards. How more points you get how less cards you can use, it all depends on the zone you are in.
You can read more about it on: http://www.urban-rivals.com/gamemodes/efc.php
Adding on the 'also same happened once I got placed'. Yes, it happens. You play against people in lower zones which leads to the fact they are able to use different cards which you might not be able to use. Same happens upwards. You are able to play against people in higher zones who cannot use cards you can use.