Amazing how the extremely smart Google gives different routes on the same search at different times.
On my first try it gave me 2 options, one was the same the blue route on your post and the second was partly like grey route but went through Nong Boa Lam Phu to Udon Thani and then down to Kalasin and it indicated very little difference in distance and only 15 minutes longer. After seeing your post I did the search again and ended up with the same you got including longer time.
Go figure.