How do the Khalkha and Buryat writing systems differ from their closely related spoken dialects?

The writing systems for Khalkha and Buryat differ from their spoken dialects primarily due to the use of the Cyrillic script and divergent linguistic conventions.1

While the spoken dialects of Khalkha, Buryat, and other Mongolian groups are closely related and share a high degree of mutual intelligibility, their written forms have drifted apart. This divergence is driven by several factors:12

• Spelling and Vocabulary: Written Khalkha and Buryat differ from one another significantly more than their corresponding spoken dialects do, largely as a result of distinct spelling conventions and differences in vocabulary.1
• Historical and Political Context: The separate political and cultural histories of these groups—such as the Russian conquest of the Buryat and the differing paths to independence for Outer Mongolia—are reflected in their specific choices of script and the development of unique vocabularies.2
• External Influences: Buryat, in particular, is distinguished from other Mongol languages by its Russified vocabulary and specific phonological and morphological features, such as the shift of /s/ to /h/.2

Consequently, while the spoken dialects remain quite similar, these literary differences create a greater divide in their written forms.12