Correct. The 1080 uses GDDR5X, which is quad-pumped and delivers chunks of RAM that are twice as big as Ethereum uses. This works fine for graphics, and for many other coins, but on Ethereum half of the bandwidth is wasted.

(bandwidth consumption is what makes Ethash memory-hard/ASIC-resistant - same principle as the tuning parameters on bcrypt/scrypt, designed to make it difficult for an attacker to scale their processing)

The 1070 Ti is basically a 1080 with GDDR5 (dual-pumped) instead of 5X (and minus one SM), and does much better, along with the 1070. The 1080 Ti has enough bandwidth to brute-force it even throwing half of it away.