I’ve noticed that when I go to Mass at different parishes, sometimes people say and do different things. Why is that?
The Mass in its current form has taken shape over the course of 2,000 years. Over time, traditions, structures, and prayers have been put into place. There are certain rubrics for the Mass that are normative wherever you go. This means you could attend Mass in Malawi or Thailand or Italy and still watch the same basic liturgy unfold, even if you don’t understand the language. There may be some cultural differences from place to place, things that aren’t specifically listed. For example, in the United States most people go up for Communion in a nice neat line. In Europe, everyone gets up around the same time. It’s a bit of a free-for-all!
Some people might take it upon themselves to change things individually. People may not like something that feels too “liberal” or “conservative,” and they may take a well-meaning stance in the opposite direction. When people (including the presiders) do things that are distracting or change the liturgy, it introduces an element that breaks the unity of the assembled community. The important thing to remember is that we gather as the Body of Christ to celebrate the Eucharist. If you are confused, ask! If you are frustrated, pray for the spirit of charity.