Adding score to a specific player using commands works as intended only when you name the player specifically (/scoreboard players add PlayerName Score 1). The result is the same when using @p and @r, but @a does something weird.

When I used @a (/scoreboard players add @a Score 1), the scoreboard does not show my username which gets the score, but rather my UUID, which looks like a lot of random letters and numbers.

Steps to Reproduce:
1. Create a scoreboard and /setdisplay to the sidebar
2. Use command

/scoreboard players add (PlayerName) Score 1

Replace PlayerName with the actual username of your player
3. Observe scoreboard value, notice that the PlayerName(username) receives the score (This part is normal but...)
4. Use command

/scoreboard players add @a Score 1

5. Notice that a new "name" is now on the list in the scoreboard, which should be the players UUID, it also does not add score to the previous player, just to their UUID.

Also, using

@e[rm=4]

will also achieve the same effect, I'm assuming that this bug was made somewhere when Mojang implemented the @e selector.

The Attached Files are pics to help describe what is going on. In chronological order the files are:
AddingScoreToSpecificPlayer.bmp
SpecificPlayerResult(Intended).bmp
AddingScoreWithSelectors.bmp
SelectorResult(Unintended).bmp