This means in his 108 games in the competition, he has hit these heights in 87 of them.
The best was in 2017 was when he hit these high averages in 18/18 of his games.
His worst was his first year in 2013 when he hit 10/18 (56%) which is still a high number but shows the progression of Mighty Mike.
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https://twitter.com/PremiumDartData/status/1092765165408395264?s=20