Using data command to modify a block. There seems to be no difference between set value and merge value besides syntax.

I am editing the count of an item in a tile entity using the data command. My command looks like this: 

/data modify block x y z Items[1] merge value {Count:1}

This just overwrites Items[1].Count with 1, which does exactly the same thing as:

/data modify block x y z Items[1].Count set value 1

Is this intended? If not, then this is a bug. I feel that the prior should add the count of 1 to the existing count in Items[1] while the latter should just overwrite it. This would allow creators to increment items in a reward chest for various accomplishments of a player, as well as other possible uses.