Barkley's 18/19 league numbers look decent...until you realize that the 3 goals and 3 of the 5 assists all came in October 2018, when he had that purple patch. If you take that out of the equation, it's 0 goals and just 2 assists in 1,049 minutes.
That highlighted part is just an assumption. There's no guarantee he will give you 10 goals and 10 assists per season just you because you double his returns based on the minutes. Barkley played 2,287 league minutes in 13/14 and had 6 goals and 2 assists; 3,086 league minutes in 15/16 and had 8 goals and 8 assists; 2,906 league minutes in 16/17 and had 5 goals and 9 assists.
Some players are just not consistent enough to hit double figures in both goals and assists, like Willian.