I have near-zero bitcoin knowledge so your post was a good incentive to search it a bit more. Albeit nothing came out, or rather nothing came into my head... Anyway, from the little I understood "Difficulty" (D) is the crux of the bitcoin madness (idiocy?): they (who?) need a solved block every 10 minutes. And so in order to keep this fixed, D is adjusted every couple of weeks in order to keep the number of solved blocks constant and not depleting the unmined bitcoins too early (oh come on, get over and done with it, miners!). So, in my understanding you can retrieve current D from some website which keeps track of it (google difficulty charts). Or calculate it yourself by retrieving the number of mined blocks. I guess.
I *think* D relates to the length of the hash you need to solve. For some D, some parts of the hash can be skipped. It's like solving a problem with specified accuracy, i.e. number of decimal digits. More digits means more difficulty and a different solution (to the decimal digits). That's probably why you get results back from USB anyway. But they are different.
If I understood correctly, even in general terms, "crux of the madness" is a good term for all these. For example, consider this implication: someone invests in hardware to solve blocks faster. That investment will only yield results for 2 weeks max. Then D will increase in order to compensate for this panter's activity and keep the number of solved blocks per 2 weeks fixed. So it's a constant antagonism in order to get a percentage from the same fixed award-pot, to split the same pie so-to-speak. And it was K.M. who said that investment profit only comes from human labour, not technology (because sooner or later all investors
adapt adopt the same technologies, even "secret" ones).
In general, it looks to me to be like the donkey running to reach the carrot. In which case carrot's speed is adjusted automatically (by the System!) so that donkey never eats it. Clever algorithms would enable the donkey eating a bit of the carrot from time to time to keep content.