@Jeruba It’s because I took a few semesters of Russian in school and can point out several sounds that gave me difficulty, sometimes saying them at all and sometimes accidentally switching them. It took me ages for my mouth to learn a half decent Ы and can’t hear let alone pronounce the difference between Ш and Щ. Plus in Russian letters change a bit depending on if they’re voiced, unvoiced, soft or hard. Sometimes Г sounds like a Russian В, but other times it sounds like К. The rest of the time it has the decency to sound like a Г. Х can be tricky too. Even though I technically know the difference between Е, Ё and И I screw them up plenty too and I imagine I’m not the only one.
I guarantee you there are a million English speakers that say И when they ought to say Ы.
I can’t explain how it sounds when I butcher Russian pronunciation, but I definitely know a lot of my trouble spots. I expect this is the case with a lot of foreign language learners.