How can Ganymede have an Earth-like gravity without us having realized it?

Imagine a small primitive humanoid civilization that developed independently in caves under the surface of Ganymede. We can assume there's enough light that filters through the crystalline surface to support life, and that there's enough air trapped in these caves for them to breathe.

But let's say these people also happen have a gravity that's slightly greater than Earth's. How could that be the case? And why wouldn't Earth's astronomers have discovered that before now?

Also, are there any other significant factors that would make it difficult for Earth-like life to thrive? Things that would be harder to hand-wave away?

(The SF here is about as hard as cotton candy, so answers don't need to be completely realistic. I'd just like to avoid directly contradicting known observations any more than I need to.)

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How can Ganymede have an Earth-like gravity without us having realized it?

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