Having spent many years all across northern WI, I've noticed that, in general, the lake temps and summers are cooler in the "Northwoods" area of Iron, Vilas, Onieda, Forest, & Price counties vs the "Indianhead" part of northwest WI in Sawyer, Douglas, Washburn, & Burnett counties.
I think this is due to two things....for the most part, the terrain in the Northwoods area is mostly hardwoods and rocks. But move west and it starts to change to sand and pine trees. I would say the Hayward area is a mix, but once you get to US 53, it's mostly sand and pine trees heading west, which I believe makes things warmer.
Also, a NW wind is common, and north-central WI is far more likely to be impacted by a NW wind blowing across Lake Superior than in areas to the west. Whether that's lake Superior cooled temps, or just an increase in clouds, it believe it leads to cooler summers and cooler lake temps than areas to the west.
What have you observed?