More that they have certain problem domains where they might pick a given search engine, and that those problem domains are biased toward using Bing for easy searches and Google for hard ones.

I'll frequently search for airplane tickets on Bing, for example, while I won't even go to Google for that. But when it comes to looking up obscure programming stuff, I'm not going to bother searching on Bing, I know it'll suck.

"More that they have certain problem domains where they might pick a given search engine, and that those problem domains are biased toward using Bing for easy searches and Google for hard ones."

That makes no sense. Why would you, or anyone, alternate with search engines if one of them too often fails to deliver? Why not just use Google all the time and avoid the cognitive overhead?

Does Google do a poor job, or a worse than Bing job, with airplane tickets?

I use DDG except for local sites, which it doesn't index anywhere near as well as Unca Googs.

They might try the query on Bing, fail, and then try the same query on Google. A large portion of those are likely to fail as well, just because the queries are hard.

It's less likely that someone will try a query on Google, fail, and then think "Maybe Bing will do a better job with this."

Well in that case wouldn't bing's percentage of failed searches be even larger than Google's? The stats don't reflect that

It would cause Google to have a relatively larger proportion of "difficult" searches. Many "difficult" searches on Bing will be repeated on Google, but not the reverse.

If you assume that all "difficult" searches fail on both engines as a first approximation, that would mean that Google will fail all the "difficult" searches, while Bing will only fail the "difficult" searches made by people who primarily use Bing.