Good question
I'm not sure myself but will ask around.
Awesome, thanks! Also, my statement was not entirely accurate: the DST root which has signed the Let's Encrypt intermediary also uses the X notation. I had forgotten about that until now.
I assume that someone thought the X sounded pretty cool, so they went with it, but I'm sure there's an actual backstory as to why.
There's no policy against having multiple registrations with the same
email address. I'd point out that there is however a rate limit for the
number of registrations for a given IP within a fixed time frame (500
Registrations per IP Address per 3 hours).
Indeed. I'm not anywhere near hitting any of the rate limits, especially the registrations-per-IP limit. It's just that the different systems I maintain use different ACME clients with different storage formats for registration data. I could move the account key from one system to the next but it'd require manual rewriting of config files to make everything work. In my case, it's easier to just run the script for the first time and have it generate a new key. Since I'm exceedingly lazy and there's no particular technical or policy reason not to do that, I think I'll just stick with that. 
On a related note, I wanted to thank you guys for permitting anonymous, automated registration rather than requiring one first manually register for an account on the website, provided various personal information, need to copy-paste API keys, etc. It's a refreshing change from the status quo.