Some things are very strange. Such as the tradition of different styles of cards for the bride's & the groom's relatives.
There could be just one style of wedding invitation cards for one wedding, the same style goes to both the bride & the groom's family.
Extended family details of both of them could be mentioned in the same card as, Bride's, Groom's.
Here is one possible reason, as told by Dad. There are 2 different functions for the bride's family & the groom's family. Many times, people invite their extended families to just their function & not the counterpart function. Hence, the need for 2 cards.
He did not fail to add, don't worry we will have just one common card & function for your wedding.