It will be because your Eevee's and Pikachu's Fullness is at different levels.
Like previous secondary features, Pokémon Amie has a few ways to track the progress of your Pokémon. There are three stats, each measured by five icons to determine how well they have been trained in that field:
- Affection: Determines how happy your Pokémon is with you.
- Fullness: Determines how much PokéPuff your Pokémon needs.
- Enjoyment: Determines how much your Pokémon has been played with.
Each value can be altered slightly by participating in certain things. For example, playing a game with the Pokémon can lower its hunger stat.
If a Pokémon's fullness is below 198 points when it is fed a Poké Puff, it will make three quick bites; but if its fullness is 198 to 213 points, it will slow down for the third bite, and if its fullness is above 213 points, it will make all three bites slowly.