can't trust distances

I've been cross-checking the distances for the segments I'll be walking on Jeju Island, and unsurprisingly, things aren't matching up. Naver Map says one thing, but my Jeju Olle-gil app says another. Trail segments are called "courses" on both maps, and the first three segments look like this:

Course 1: 15 km (app); 15.1 km (Naver)
Course 2: 14.8 km (app); 15.5 km (Naver)
Course 3: 21.3 km (app); 28.2 km (Naver)

The distance disparity for Courses 1 and 2 seems okay, they're off by only 100-700 meters. But the difference between my app and Naver for Course 3 is troublesome: that's almost a seven-kilometer disparity. Even more bizarre: Course 3 contains an unnecessary loop that I have no intention of doing: I'm going to follow the coast as much as possible. In doing that, the distance shrinks from 20-some kilometers to only 13.8 kilometers.

Overall, the difference in distance between my Jeju app and Naver Map is substantial: my app says the total hike will be exactly 341 km, but Naver Map says the route will be 370.8 km long. I honestly don't know which resource to trust, but since Naver shows each segment of the trail, which I can follow in real time, I'm probably going to use Naver for most of my navigation. Naver isn't perfect, but it's 99% reliable. I also assume there will be periodic signs along the length of each segment, so in theory, it should be hard to get lost, although, as with the east-coast trail, the Jeju Olle path doesn't stick to the coast, so I can't rely on navigating by always keeping the ocean on my left.

Another big question I'm trying to answer is whether I'll be able to motel my way around the island or be obliged to camp at certain points. According to Naver Map, there are plenty of pensions along the way, plus guest houses (which I'd prefer to avoid but maybe can't) and a few scattered motels, hotels, etc. Most of these spots also have restaurants and convenience stores, so I won't be starving as I work my way around the island's perimeter. So, yes: there's lodging at every stop, which means I won't need to bring my big backpack and camping gear: I can make do with a smaller backpack, like last year. Almost every stop is at a city with a small or large port, so every day will end with me back in civilization. Not sure how I feel about that, but on the bright side, I'll have places to eat and sleep.

I need to start working on a Kevin's Walk 6 blog, although that blog will actually chronicle two walks: the Jeju walk and the walk to the Andong Dam from Sangju. My boss and I are going to flesh out my travel dates tomorrow, after which I'll be visiting a travel agency to get my round-trip plane ticket. I plan to take off the usual number of vacation days since I'm doing two walks this time, but in reality, it's only 21 walking days, so I'll have about a week's worth of buffer days during which I can fart around in Seoul.