What did I do wrong?

I did my first inlay. Black walnut in a birch butcher block. All sanding, the surface is smooth. After putting on Minwax Spar Varnish, all joints in the inlay not even.

I did my first inlay. Black walnut in a birch butcher block. All sanding, the surface is smooth. After putting on Minwax Spar Varnish, all joints in the inlay not even.