When pickblocking ferns or grass and having ferns and grass in your inventory, you will get either ferns or grass no matter which you pickblock depending on which is first in your inventory. If you only have ferns or grass in your inventory, no matter which you pickblock or where the ferns/grass is in your inventory you will get which ever one is in your inventory.
If you pickblock ferns or grass with neither in your inventory, you will get the correct item.
If you use the command /fill tallgrass and pickblock that grass you will get a item that looks like grass but is called fern.