"Have guests" VS "Have guests over"

Question

I was wondering how the two choices below differ in meaning and whether they are both idiomatic here:

Sam) I can't make it to your house today Randy; I'm sorry. But I promise to come tomorrow instead.
Randy) Well, just why?!
Sam) ....................

a. I have guests today.
b. I have guests over today.

I have an intuition that when we say: "I have a guest / some guests over" it means they have not arrived yet, and I am waiting for their arrival or expect they come soon, while when we say "I have guests," it means my guests are at my place right now. This is why I think despite the intention is clear by both options below and you may confirm that they are both idiomaticaly correct here, "b" is more natural.

Answer 1

If your guests have not yet arrived you could say "I have guests coming/coming over today. It is the word "coming" that indicates they have not yet arrived.

Over does indicate the guests are at your home, instead of being invited to another venue.

If "over" is short for "staying over" it means the guests will be staying overnight or perhaps longer.

Answer 2

To 'have guests today' and to 'have guests over today' mean the same thing, but the second (adding 'over') is less formal and a regional (mainly US) variation. The guests may be present at the time of speaking, or just expected to arrive that day.