I think it's due to the fact that the human brain does not work linearly, like a lexer. It recognises patterns of letters. And once you've seen the word 'pranker', it's hard to un-see. I think it's a particular problem with web addresses because you know there will be several words joined together without spaces, so you're already starting by trying to identify clumps rather than reading words. Plus web addresses often contain made-up words.
Or it could just be that these days people just scheme through text. Just reading from left to right, you'd auto complete 'startup' and honestly without another 'p' ranker auto completes itself too. Which is why I am inclined to think that people are just scheming through words now.
The brain does not read linearly from left to right, letter by letter. Unless you're 3 years old, you see a word and parse - or rather, pattern-match - it as a whole. And the matches depend heavily on whatever is in your head (that's how context and anchoring work).
I too, initially, read it as "startup pranking", even though there's only one "p" in the domain name.
I'm not sure that 'scheme' is 100% the word you meant to use (perhaps you meant 'skim'?).
People don't read from left to right. The same way that they don't press individual letters on the keyboard (think about it, if you're typing at 100 WPM you can't think of every single individual character at that speed).
A perfect example is Spritz. Try this. I managed to comprehend at 700 WPM. No way my eyes could move at that speed. It works because of pattern matching.
If a lot of English speakers parse the address incorrectly, then the fault lies squarely with the address. "You people are reading it wrong!" just doesn't fly :)