The courts are so overrun all the time, you can get a VERY reduced sentence simply by pleading guilty and helping them avoid the need to have a trial. Especially if you have any sort of defense at all.
If the woman didn't have clear, conclusive proof that the husband had poisoned her, or the D.A. felt like his evidence wasn't rock solid enough, then I can see how the perp might be able to use a guilty plea to knock the sentence way down.
But the suggestions that he might have connects or just be getting off easy because he's white or because "domestic issues" get light sentences are all valid hypotheses too.