Some OC lexical doublets have different vowels. Such pairs are sometimes called “ablaut pairs”. What process is behind ablaut is so far not known.
This post discusses a doublet form for the numeral ‘100’, which suggests that one of the processes, at least, behind ablaut, is interaction between the first and second vowels of a lexical compound, such that the first vowel assimilates the second one. Subsequently the compound’s second morpheme individualizes with its new vowel.
Baxter and Sagart (2014) reconstruct the word for ‘hundred’ as 百 OC *pˤrak > MC paek. There is a rare word 皕 ‘two hundred’, not attested before Han. whether in texts or palaeographically as far as I know. The Shuo Wen says it is pronounced like 祕 （pijH < *-it-s), while the Guangyun reads 彼側切 pik. An alternation between MC -it or -ijH and -ik indicates OC *-ik. B&S (2014) did not reconstruct that word but would probably have given OC *prik. The *i vowel in ‘200’ alternates with the *a vowel in ‘100’. It happens to be the same vowel as that in 二 *ni[j]-s ‘two’. It is possible that *prik arose out of 二百 *ni[j]-s pˤrak, with the vowel of *ni[j]-s spreading rightward, replacing the original low vowel in the ‘hundred’ morpheme, thus *ni[j]-s pˤrak > *ni[j]-s prik and ultimately 皕 prik, as the vowel sufficed to distinguish 皕 from 百 .
Added Sept 18, 2023: not just the vowel of 二, but the [-pharyngealized] feature appears to have spread from 二 to 皕. This is in agreement with the observation, made in a Chinese PhD some years ago, that in disyllabic words found in early Chinese texts, the two syllables agree in being both type A [+pharyngealized], or type B [-pharyngealized] . It causes me grief that I have misplaced the author’s name and the dissertation’s reference. I would be grateful if a reader could provide this information. Meanwhile, the pharyngealization harmony in OC disyllabic words, including in the case of 二百 > 皕, can be explained by a process copying the pharyngealization feature of the first word onto the second, overriding the second word’s original specification. This would go some way towards explaining the frequent alternation between pharyngealized and non-pharyngealized cognate words within OC word-families.
Reference
Baxter, William H. and Laurent Sagart. 2014. Old Chinese: a new reconstruction. New York: Oxford University Press. 4