Because it was the fifth patch, not the ninth patch...?
v0.5 means Pre-release (0.X.X), Fifth Patch (X.5.X), no Hotfixes ( X.X.<N/A>(or 0) )
It does not mean "My game is exactly halfway there!"
Again - I don't say you SHOULD choose one naming system over another. I'm saying, people subconsciously perceive these numbers as progress. Could you use it for your advantage? Yes. Should you? No. Many things are like that - optional.
You can communicate with you audience. Is it mandatory? No. You can ignore them.
You can inform your comunity about your progress. Is it a must? No. You can write "Done when it's done".
You can consider some of community ideas. Ot you can ignore their claims.
You can use some methods and instruments or you can ignore them.
Everything is optional, everything has consequences. One way or another.
As for your "v0.5 means Pre-release (0.X.X), Fifth Patch (X.5.X), no Hotfixes ( X.X.<N/A>(or 0) )"
Sorry, it means nothing. What's in your head is in your head until you explain it. In this particular example you can ignore all the numbers and just name it "Pre-release, Patch 5" - it will be more clear than v.0.5.00. For the audience I mean. As a part of said audience I can see this is not a release, but pre-release, and this is fifth patch. How much till the complete version? No clue. And if you add 17 more patches - it will look fine. Pre-release, Patch 23 look normal with no content. But 1.23 with no content raise a question - where is content if there is "1." in the version?
Shitpurple changed his project version nsming twice or thrice. Was there any reasons? Perhaps. Did that make things clear? No. What audience saw is numbers changing, no new content added. That means something is not right snd someone trying to do some wierd shit. So overall impact was negative. But of course he can rename his versions even everyday - it is his project after all.