Complex sentence: They both remained in the alphabetical repertoire because they represented different numbers in the Cyrillic numeral system, eight and ten, and are therefore sometimes referred to as "octal I" and "decimal I".
Split into simpler sentences: They both remained in the alphabetical repertoire because they represented different numbers in the Cyrillic numeral system, eight and ten. They are, therefore, sometimes referred to as "octal I" and "decimal I".