I bought a 08522-B1010 replacement horn; why are there two different bolts?

I scanned the Japanese language install instructions through Google translate, and they confirmed they are supposed to be two different sizes: M8 x 25mm and M6 x 30mm. Also, the M6 has a Philips head.

Of course, the instructions aren't meant for a Gen 5 Prius, but the install is for the high pitch horn to be on the left, with the M8, and the low pitch to be on the right (in place of the original, if I'm interpreting the install videos correctly) on the right with the M6.

I suppose I can reuse the one existing bolt, but are there really two different bolts involved here--are the attachment points different sizes?