Sometimes double crowns were supplied on large sheets which were included with various publicity material. One printed sheet would have several double crown posters on it and then the onus would be on the cinema owner to cut out the double crowns. So if said owner's cutting skills were a little unwieldly, that might explain any uneven borders etc.
You do also find double crowns which are, as you say, double bills cut in half. So either one of those options could be responsible for the size of your Tarzan poster
