My dad and mom had one of these at home (neither were accountants) and I definitely spent too much time playing with it!

This post has the answer. The finish that you used is causing the color difference. Mineral oil, like any oil finish, soaks into the grain of the wood. It makes it look wet, deep, and sometimes almost translucent. Think about how a drop of oil looks on a piece of paper, and then imagine that soaking in through all the wood fibers. Mineral oil is colorless and doesn’t amber like linseed oil does, but the depth it brings makes the wood dark.

The finish IKEA used is probably a film finish made of some kind of plastic. This post mentions a UV cured finish. It’s likely on the surface of the wood only; these finishes can be crystal clear, adding no color, which is great when wanting a cooler-hued tone to natural wood or to preserve the bright, light color of light colored woods. They might not soak in or make the wood look wet. This accounts for the color difference.

The good thing about mineral oil is that it doesn’t cure. It’s also not super resistant to water. So with use and regular cleaning, it will eventually wear or wash away, and the countertop will look lighter and even dried out. The way my cutting boards and wooden spoons get when they need oiling! Once the countertop is all dry and thirsty looking again, OP can choose a more suitably matched finished than oil was.