Is it not more likely that a big enough mining farm mines to a lot of different reward address, making it appear as many small solo miners?

Is there ANY indication that it isn't the case? It sure does look like the sun is going around the earth, but there is another explanation.

Even if the said-to-be-different-miners were connected with different IP addresses the whole time, invalidating the possibility of a change of IP address between blocks found, I would still suspect one farm with a bunch of IP addresses. That is so much more probable.