Learn English – When is “Next Saturday”

In British English, 'a week Monday' means one week after the next Monday. I'd be surprised if Australians were using it any differently.

So assuming you received the email on Saturday 4th July, they want to interview you on Monday 13th July. However, you should still email them back to confirm that this is the correct date.

American English has a tendency toward a certain overflow of prepositions (sometimes attached to the verb, sometimes without).

It's the same phenomenon as in phrases such as,

— Come on in (vs. "Come in")

— I am basing myself off of those findings. // He has weaned himself off of alcohol. (in both cases, "of" can be left out)

You can "take the dog for a walk" and "take the dog out for a walk"; these phrases are equivalent.

So there are a lot of parallel constructions like this; American English will generally have more of them than BrE. The choice is basically up to the speaker/user.

It comes down to what kind of phrases you've heard more often and have accustomed yourself to. We learn language (even our own, when we were babies/toddlers) by aping and repeating whole phrases, not just words. I find it useful to have a notebook where I don't just write down words from the language I'm learning, but whole phrases (including these cases of un-obvious, redundant use of prepositions).