Well, there is no "easy" answer, because we haven't isolated all the genes, so can't say for certain what is going on with each and every horse.
HOWEVER - the "easy" answer is that roan and sabino (at least, sabino 1, the only one we have isolated) are mutations of the same gene. Also included on this gene is tobiano and all the DW we have found so far. They are all mutations of the KIT locus. So it is not surprising to see similarities in the way the genes express, as well as differences.
Let's have a look
Puchilingui, a DW TB stallion. He is tested and verified as DW.
A tested sabino 1 mare.
Tobiano with "cat tracks" and haloing.
"Classic" roan (and fricking gorgeous!).
As you can see, these all have similar phenotypes when you are looking for similarities
Same as classic bay, brown and wild bay, or pearl and cream.