It is strange behaviour but people often will do it because they expect that such a website exists. There's a similar effect when people have a search engine set as their homepage. They will enter the url of the site in the website's search box rather than on the browser's address bar.
Direct Navigation is a very real effect though. However it was related to the size and growth of .com TLD. There seems to be a difference with Direct Navigation in ccTLDs though because of how people remember ccTLD URLs and of course there is a big difference in traffic.
There are many generic/Direct Navigation keyword domains in .co.uk as well as most other ccTLDs. However with .com, .de and .uk, it is like fishing in an ocean. With some of the smaller ccTLDs, it is like fishing in a puddle. The growth of ccTLDs and the inward focus of many countries with strong ccTLDs has affected .com Direct Navigation. It is just a theory but I think that some of the decline in PPC on parked keyword domains over the last five years or so could be due to shifting registration patterns and the rise of the ccTLDs.