It is very surprising to find out that all of these companies are operated by the same parent company. With each of these companies having very different brand messaging and selling points, targeting a wide range of consumers, it is incredibly surprising that a single company operates all of them.
In the modern era, most major corporations are owned by about six parent companies. The big companies that come to mind are Disney, NewsCorp, G.E., and Comcast, though there are others. These parent companies rule over a vast majority of major corporations and media outlets in the United States. So to put it plainly, I'm not surprised the dating sites are owned by the same company.
It's not surprising at all, since all of them are dating apps, but cater to people seeking different types of dating. Tinder, for example, is more for hookups. OKCupid, Match.com, and PlentyofFish are more for people seeking longer-term relationships, but each has a slightly different way of approaching the process.
It's not surprising. Someone figured out how to make money off of dating, meetups and cheating. There's an audience for all of those things, and the company figured out how to capitalize on them, and how they might relate to one another. It's all about degrees of relationships and profit from that.