There isn't anyway to confirm based on photos like that a particular crepe myrtle cultivar. Big box stores are not really the place to shop for specific varieties. It seems like their garden center is there more of a convenience to supplement their other home improvement merchandise and thus ensuring accurate tags on their trees or guaranteeing them to be true to name is not too high on their list of priorities. Instead they want to sell large quantities plants that are the most in demand and also inexpensive to produce. If you want to be picky (and you should be) big box stores would probably be a last resort.