That's because the triangle displacement isn't the same size / half the size of your other displacents; the lines of your small displacement don't line up with the ones of the larger displacement. You need to adjust the size of either displacement so that the smaller one is either as large as the larger one, or about 1/2 of the size. This way, the lines will match up. A good thing to do is make all displacements the same size, say, 256x256. If you have to place a smaller one then, you can use 64x64, 128x128, etc. That's my personal way of handling displacements.