The engine will use what resources are available to improve the experience.
You may be getting a higher frame rate, or the graphics driver may be spinning in a busy-wait loop waiting for the frame refresh which might count as “using the GPU” in windows instrumentation.
To figure out what it is, there are various modes you can change – draw everything with a simple shader, or draw everything with a small texture, or use a small window (320x240) and so on.
If one particular thing changes the amount of CPU used, you know that that’s the reason (e g, resolution → fill rate on the card.) If nothing changes the usage much, then it’s likely something more system-dependent, like the graphics drivers themselves.