Why 2 stream threads are handling events with same key?

2718-3-1772 is the key for events, 4 events with same key, but they are handled by 2 stream theads, why? these 4 events are in same partition, right?

see attached 2818-3-1772 for events:

we have one topic, 12 partitions for the topic.
8 application instances are running .