Almost all translations of just about anything will do this (or subtract words in some cases). In the case of Avestan to English, you're going from a heavily inflected language (meaning a lot of meaning is implied within the structure of the word) with a significantly smaller vocabulary to an uninflected language with strict word order, more near-synonyms, and often without an exact 1:1 translation for some words.

Even if you committed to a set of 1:1 vocabulary, where some words might not make sense in their English context all the time, simply translating each word in the same order, without adding anything, would result in an unintelligible mess. Avestan just doesn't consistently employ the same subject-verb-object construction, and (like all fully inflected languages) the ending applied to a single word can translate to whole phrases in English.

Many of the same issues come up with translating Avestan into Modern Persian and other languages that have lost their inflections over time.