There could be a number of reasons, but there isn't much to go on. It could be:
-the in game settings.
-the resolution.
-the game engine.
-the cpu.
-the ram.
-the storage.
-the Gpu Boost algorithm isn't comfortable with the gpu's parameters with the new power limit in place. Ever since it was introduced with the GTX 10 series, OC'ing hasn't really been the way to go with these cards, as Gpu Boost will reign in almost all custom settings if it 'feels' you're pushing it too far. So if the card runs hotter than it did previously, or it's running into the power limit more frequently, Gpu Boost is going to respond in kind. Nvidia more or less killed gpu OC for more casual users.
-a combination of the above. Some of them go hand-in-hand with one another.