I agree that it could be a matter of who has tested. It could be that one side had smaller families or the families did not thrive. It could be a financial consideration where one line has tested more due to financial ability to afford testing. Or you may not be recognizing the matches because the surname died out due to a lot of girls in the family.
I agree about the recent immigration issue. Ancestry hasn't tested outside of the U.S. yet. Also, if the message boards are any example far more southerners in general appear to be testing than people from the north. I look at the trees of many of the message board posters and there is a striking difference in the amount of testers with trees with southern roots compared to trees with northern roots. I don't know why and it could be the message board posters don't represent the whole very well. There may be other population imbalances or demographics that could play into something.
Also, I do think there could be a difference based on gender but not for the reason that one might think. Men can receive results for Ydna, mtdna and autosomal where women can only do the later two. A lot of emphasis has been on Ydna for surname studies and Ydna has been done for a longer time period. It may just be that the men were also among the first people to be tested with ancestry for autosomal. That may change since autosomal opens some doors for women. I know that I did not test earlier because all that I saw offered was Ydna. If I was not a regular with ancestry I might not have realized what other options were available for women.
Also, both discrimination issues or expectations could be a factor. I have spoken with some people who will not test because they feel their heritage is lost, such as a Jewish man whose family died in the holocaust. He doesn't see much chance that any relative close enough to be recognized would have survived to be tested and even if they did the records to provide names may not have. That is just one situation I am aware of. Some cultures handle ancestors in different ways and there may not be as much interest in one side compared to the other.
NPE's are frequently thrown out as an explanation for an area of confusion. I am curious whether anyone thinks there is a scenario where NPE's could explain a situation such as this one where there are matches to the father's line. That is other than adoption or the very recent surrogate situation.