According to instruction set, the AVR MUL (unsigned) instruction copies result bit 15 to C bit.
I can't really see the reason for that, except (requiring further investigation and I am not really sure about it) when using number format 1:15 = 1:7 * 1:7, even so, it seems very strange, since 0xB6 x 0xB6 turns on bit 15, and not all 1:7 x 1:7 does that, 0x85 & 0x85 does not.
Can you see any reason and advantage to have result bit 15 copied to C bit?
What one would use this carry in the subsequent programming?
I think N bit would be more fashionable to represent bit 15.