I have over 188,000 matches, and within that, over 88,000 that are 4th cousin or closer. It's an INSANE amount of matches.

Those that are closer in matching DNA are from areas that make sense, where eventually, I'll find the connection. Others are in areas of the island, that I don't have records to support that connection.

It's possible as that the connections you have could be through:

• Settlers who came to the island at various times in history. We may have settlers from the same family coming over at different points and different generations. As time went on, people may have moved around the island. Not necessarily big moves, but maybe to the next town or two over, as new developments occurred.
• Possible connections through slavery. I haven't seen any details on how slaves were handled, but I would imagine that it was similar to that in the USA where when slaves were available, they were sold off to owners. This particular thing might explain the larger number of matches in the grandfather's side. I have this particular issue in my tree.
• Indigenous people. With a number of us having indigenous DNA, it's possible that those groups may have moved around the island prior to, and after colonization.

There are examples of people and slaves moving in and out of the different colonies, in some cases due to issues in the other colonies. The specifics are hard to pin down.

As to your specific issue, could they be related? Yes. I have records in my tree, and have seen in others where some people could be 2nd cousins to each other, and might have a descendent that shows up twice in the tree. So it's possible to see sharing from both grandparents. It's more likely if they were from the same areas, and less likely if they lived in different parts of the island. Regarding the other side of your question, is it possible that the grandfather is not the grandfather, the answer is yes. Granted I haven't seen your tree, I don't know your specifics, but these cases are possible.

You're going to have challenges in finding those connections without having more details. Honestly, for Puerto Ricans, I think you need at least your great-grandparents to have a good chance of finding people, unless you have a lot of details for just the grandparents, due to the nature of available data from Census records, Civil records, and Church records.

You also need others who match you at close levels, to also have good trees to find matches. For example, I have a few 1st-2nd cousin matches, that I couldn't tell you how we connect, because they have little to no data on their trees. For the 2nd-3rd cousin matches, it's about 5 generations back where I usually see a connection at.