Typical digital attach rates are around 10% and people tend to think that people with Nintendo consoles buy digital less...Either everyone predicting 500-600k digital didn't do the math or they think that MK8 has a pretty giant digital attach rate

270k digital would be 10% of 2.7 million.

MK8 is not a typical title. It's by far the most hype game the Wii U has had and will have. It's not unreasonable to think double the people bought it digitally compared to your average game. 

Why would more hype lead to a higher proportion of digital sales specifically?

it's a game fans will know they will love and won't be trading in 2 weeks later. FOr example I will buy GTA 5 digitally but if I was to ever get say Destiny it sure as hell wouldn't be digital unless it was a flash sale at bargain price. Minecraft for example has a very high level of sales that are digital. Cheaper title but same logic. 

But on the other hand, many fans will want to own a physical copy both to display it on their shelf and in case their console dies.

an those fans will always get physical. The digital crowd will get digital plus another good portion woh only get digital for big titles they know they'll love. 

I say undertracked by around 250K.

I don't think MK8 would convince either digital or physical buyers to change their preferred method of purchase. It's not like there was extra incentive to get it digitally rather than on disc, at least not in any way that doesn't apply to plenty of other Wii U games.

i think it's a case of a 150k under track on here, games sitting on shelves and decent digital sales.