Unlike most other stats, Evasion is a stat that is no different for any Pokémon. This helps to determine if the opponent will land the attack it's attempting to hit with in battle.
The higher the defending Pokémon's evasion, the better the chance it has of dodging. Conversely, the lower the defending Pokémon's evasion is, the less likely it will dodge the attack.
Evasion is affected by many factors, from certain abilities to the various moves that a Pokémon can learn.
Starting in Generation VI, Pokémon with the Keen Eye Ability will also ignore Evasion boosts by opposing Pokémon.
The Evasion stat is a hidden stat used to possibly prevent opponent's attacks from hitting.
So, from what this says, depending on what Wingull can learn and its ability (although none of Wingulls abilities can raise its evasion) are what determines this.
Or, maybe it's just luck that Mareep and Tauros got hit. Razor Leaf does have 95% accuracy, so it's not guaranteed to hit all the time.
Hope this helps :)