In previous versions blocks like grass and stone (blocks which use alternative block models) used the normal block model, rotating it 90, 180 and 270 degrees in order to create a completely random arrange of textures (see attachment 1.png).
However in 17w48a the default resource pack uses only the normal model and a 180-degree rotated one, creating a specific pattern (can be seen in the circled areas of attachment 2.png).
The same goes for any block that uses this method:

{
    "variants": {
        "normal": { "model": "name_of_defaul_block_model" }
    }
}

(See grass.json)