scoreboard players add Player <Objectives> 1
will make same effect as
scoreboard players set Player <Objectives> 1
after put the first command into commandblock
it set my score to 1 not add 1
scoreboard players add Player <Objectives> 1
will make same effect as
scoreboard players set Player <Objectives> 1
after put the first command into commandblock
it set my score to 1 not add 1
Cannot reproduce.
What was the score of "Player" BEFORE you executed the command "scoreboard players add Player <Objectives> 1" ?