The second selector in the /stats command does not work with selectors, but does work with strings/names.

To reproduce, first set up an objective and show it in the sidebar:

/scoreboard objectives add test dummy
/scoreboard objectives setdisplay sidebar test

Then we can use a second selector with the command.

/scoreboard players set @p test 0
/stats entity @p set SuccessCount @p test
/testfor @e

If using spigot you will now see a final score of 0, but if using vanilla you will see a larger number as the score.

To show it working with a simple string, we can do the following:

/scoreboard players set fakename test 0
/stats entity @p set SuccessCount fakename test
/testfor @e

The final score will be a number larger than 0 when using spigot and in vanilla.