Read as a system, the Mongolian consonants are organised around voicing in some places and not in others, and the alphabet records the pattern exactly. The bilabial stops pair, voiced and voiceless. The dental stops pair, though the script does not always distinguish them. The affricates pair. The sibilants do not pair, and the two the language has, plain and palatal, differ in place of articulation rather than in voice. That asymmetry is common enough across languages and it has a consequence for a borrowed alphabet: a script built around it cannot write a voiced sibilant without an addition, and Mongolian eventually made one. The pattern is also uneven in what the writing marks.
Where a distinction exists in the language the script may nonetheless leave it to the reader, as with the dental stops, and where a distinction does not exist the script may still supply a letter, as with the aspirate added for Sanskrit. An alphabet is not a phonemic inventory, and treating it as one leads a reader to conclusions the language does not support. It is a set of decisions about what is worth writing down, taken at different times by people with different needs. The voiced sibilant is a good test of the point, since it is a letter for a sound that was not in the language when the alphabet was borrowed and is in it now, met in a large body of ordinary loanwords as well as in learned vocabulary.