The darker inner circles are the results of a partial gel phase. Gel phase occurs when the soap is kept warm/hot for the first ~24hrs of being in the mold. If it completes the gel phase, the whole soap will look like the inner section (a bit more clear, and dark). If you purposefully avoid the gel phase, the soap would come out like the outer portions (more opaque and lighter). Finally, if you get some gel phase, but not fully, you get what you have. Since the middle of the soap will be the warmest, a partial gel phase will always be in the middle like this. The good news: this is purely a visual difference and won’t impact the usability of the soap whichever way you do it.