Not quite. ALL brake fluid is synthetic. It's just that some manufacturers make a big deal of it and put the word "synthetic" on the bottle.

I haven't heard of any technological improvements with respect to "hygroscopic technology". There are minor variances between manufacturers.

DOT 3, 4, and 5.1 are all glycol-based and compatible with each other. The numbers denote the boiling resistance of each DOT grade. I'm not sure about the difference in hygroscopic properties between the different grades. I've heard that the higher grades actually absorb more moisture than the lower grades.

DOT 5 is silicone-based, and is incompatible with the other DOT grades.