Your idea that the subjects are not taking the question seriously is a good one.

I had a discussion with someone about a very similar real life 'Linda'. It was finally resolved by realizing that the other person didn't think of 'and' and 'or' as defined terms that always differed and was quite put out that I thought he should know that. To put it in 'Linda' terms: he know that Linda was a feminist and doubted that she was a teller. This being the case the 'and' should be thought of as an 'or' and b was more likely than a. Why would anyone think differently? It kind of blew my mind that I was being accused of being sloppy or illogical by using the fixed defined meaning for 'and' and 'or'. I have since that time noticed that people actually often have this vagueness about logical terms.