When numbers become speech, it's like two worlds colliding. One is the elegant and straightforward logic of mathematics; the other is the mess that we call language.
In English, for instance, the decimal system, for some reason, has to make do with vocabulary based on a duodecimal way of counting (eleven, twelve); Germans insist on reading their two-digit numbers backward (three-and-twenty for 23); and don't even mention the French with all their "sixty-eleven" (71) and "four-twenty-twelve" (92) acrobatics.
And what about Japanese? Well, the good news is that it largely does without comparable quirks in number reading. The not-so-good news is that it has a number of other issues with numbers, one of which is its system of numeral classifiers, or 助数詞 (josūshi).