Внимательно читаем то, что написано жирным
The ASCII character set defines characters in the range 0x00 – 0x7F. There are a number of other character sets, primarily European, that define the characters within the range 0x00 – 0x7F identically to the ASCII character set and also define an extended character set from 0x80 – 0xFF. Thus an 8-bit, single-byte-character set (SBCS) is sufficient to represent the ASCII character set as well as the character sets for many European languages. However, some non-European character sets, such as Japanese Kanji, include many more characters than can be represented in a single-byte coding scheme, and therefore require multibyte-character set (MBCS
Many SBCS routines in the Microsoft run-time library handle multibyte bytes, characters, and strings as appropriate. Many multibyte-character sets define the ASCII character set as a subset. In many multibyte character sets, each character in the range 0x00 – 0x7F is identical to the character that has the same value in the ASCII character set. For example, in both ASCII and MBCS character strings, the one-byte NULL character ('\0') has value 0x00 and indicates the terminating null character
A multibyte character set may consist of both one-byte
characters. Thus a multibyte-character string may contain a mixture of single-byte and double-byte characters
. A two-byte multibyte character has a lead byte and a trail byte. In a particular multibyte-character set, the lead bytes fall within a certain range, as do the trail bytes. When these ranges overlap, it may be necessary to evaluate the particular context to determine whether a given byte is functioning as a lead byte or a trail byte.
Иными словами в вашем случае
"Я" - является MBCS
(не путайте с Unicode) строкой, где символ 'Я' - занимает 2 байта, а символ '0' - один байт.