In English language a lot of complex words and forms exist which are not so commonly known by native English speakers as well. Only learners of English whose keen mission is to understand the grammatical aspects of language are aware of such things.
When a word or a group of words is omitted from a sentence without changing the context of the sentence, it is termed as ellipses. When an ellipses occurs, it is understood that the word or words are missing and still the meaning of the sentence is clear.
Jess has four dogs; Allison, three.