SPARQL - FILTER NOT EXISTS vs MINUS

Could anyone please explain to me the difference between FILTER NOT EXISTS and MINUS in sparql. Specifically, when they don't contain WHERE variables in their body. For example why does the following query returns anything?

SELECT * {
   ?s ?p ?o .
   FILTER NOT EXISTS { ?x ?y ?z } .
}

and why does the following query returns everything?

SELECT * {
   ?s ?p ?o .
   MINUS { ?x ?y ?z } .
}

Thanks in advance