With most things, trends just happen with little rhyme or reason. Someone saw it, liked it and did it themselves. Look at the trend of icons where there were no eyebrows, or the super gaussian blur (see mine for example). It's called mob mentality. People see other people doing something and feel like they have to do the same. Peer pressure, ftw.
I know, personally, I prefer uniformity in the comms that I've modded. I'd want either IC or OOC but I don't want to see both in one place. It throws me off to see peoples names and then suddenly ~fatcat thrown into the mix. And there have been times where the people that have had the OOC names in a comm of majority IC journals aren't the best role players (I have many, many years of experience to back up that statement so I'm not just saying it because of one or two people but a whole slew of them), so it can show a sort of "maturity" in a role player.
That being said, there are times when OOC is totally appropriate (and often the rule): - college/high school/young adult games where the characters range from 15-25 (maybe 30) will ask for OOC because the journals are more of a representation of their blogs/tumblrs/etc. - specialty games (supernatural/mafia/etc) games, like your example with ~thatoldblackcat. - fandom games (movies/tv shows/comics/books/etc) where the name of that character was snatched up immediately and is likely not even being used, so your only option is to get creative with it.
As for the fragmented versions, I like to do that in a way that I can re-use the journal sometime down the line if I need to. This journal is a last name. It can be male or female. Another journal I have is ~milro, where the characters name is Emilia Rose, but I can easily change that so something like Mildred Roman, or Michael Lance Roads.
It can also be laziness winning out. If someone has the option to type out 16 letters or 6 letters, which one do you think they'll choose? More often than not, they'll go with the 6, right? Easier to remember, harder to screw up. And if you do screw it up, you're not staring at it for a while trying to figure out just where you went wrong.
Ultimately, it's personal preference that decides whether that rule is going to be enforced. Some mods are more lenient that others. And don't be surprised if a few years down the line its the other way around and OOC is what is in demand.