It could be to do with something called “ablaut reduplication”. Very basically English has a - kind of - untaught sound order that native speakers inherently apply to the language. Wikipedia will have an article to explain it better. Specifically the vowel order I-A-O. A great example is the phrase “Bish bash bosh” which is getting coverage recently. (One notable exception is “shit, shower, shave” but that is probably down to the chronology of the actions.)
Go further. For example, people say 'gypped' without knowing it's a pejorative reference to the word 'Gypsy' which is itself a pejorative of the Romani.
I've definitely had a similar feeling with band names and brand names, etc. You're just so used to hearing them that they are their own thing without being the component words that the name contains.
I think the pronunciation, specifically the blending of the end of "upside" and beginning of "down", turns it into one of those compound words that your brain interprets as an independent word, rather than a combination of its composite parts.
Unused to wonder if the radio announcers that are always reciting the station call letters found that the letters stopped sounding like individual sounds, and the whole recitation became a sort of "word" for them. Like "You're listening to 102.9FM WBLM!" Did it stop being "double-you bee ell emm," and turn into more of a mashup of "dubbleyabeeyelmm"?
True, the difference is pretty subtle, especially to a listener, but I wonder strange things sometimes...
I think this is the case for a lot of words. It ceases to be a combination of words and it's just one word. Then in the shower you break it down and ohhh.
And then someone tries that again with "Just realized that "downside up" means "the down side is up", making it downside up" to see if it makes anymore sense.
Apparently "W" was originally written as "uu" as early as ~600AD, hence the name, however it still used Latin/Roman letters which hadn't yet distinguished between u and v as letters. For at least 700 years, u and v appear to have been considered the same and interchangeable (so "Double U " could look like "uu" or "vv") but it depends on your language whether it was verbally called a "U" or a "V" until the first recorded distinction between the two in a Gothic era alphabet written in 1386. The two apparently did still see some overlap in use until about the 1700s with the turning point appearing to be when the distinction between their capital forms was accepted by the French Academy in 1726.
tl;dr: "Double U" predates the distinction between "U" and "V" so it's up to chance which letter a language called it before it stuck.
IIRC from high school, they taught us “V” was “Vega” and “W” was “doble Vega”. Looking at Wikipedia, I may be remembering that wrong. They have “ve” and “doble ve”
(Ps I also have these thoughts about breaking words down (unicorn is uni-corn) and some people get really snarky about it. Don't let bad comments get to you.)