why deixis is ambiguious than reference?
I'm not sure what you are asking. Deixis is about the position of the speaker. So, for example, Max and Sam are sitting across the table from each other. There are 2 cups on the table. Max asks Sam to use "this" cup--the one closest to Max, not "that" cup--the one far from Max.
Come here (to the speaker)
Go there (away from the speaker)