Hello Justin
What you state makes perfect sense when comparing the MAC address table with the routing table or even more with access lists or route maps. These entities are searched sequentially and when there is a match, the search stops, and action is taken.
However, the MAC address table operates differently, and this has to do with the type of memory in which it is stored. Content Addressable Memory (CAM) is type of memory where the MAC address table is stored, and it is also why it is often referred to as the CAM table. Where the previously mentioned entities are searched sequentially line by line, and each line corresponds to a single cycle of the CPU, the CAM table is searched in full in only a single CPU cycle. This is what makes switching so fast. The other characteristic of the CAM table, that actually answers your question, is that it only makes an exact match. Only when there is no exact match will it match with the general multicast address group used for IGMP. You can find out more about CAM as well as TCAM memory in the following post:
I hope this has been helpful!
Laz