Why Future Continuous ?

[msrdaria]

"Sonia is usually at the gym between 6.30 and 7.30. There's no point phoning Sonia now. It's 7.00 and she will be working out at the gym.(work out)"

I have this example from the exercise where I need to choose between Future Continuous or Future Perfect.
I understand it's not Perfect, and so I have to choose Future Continuous.
But don't understand why.
Why it is not Present Continuous instead?
How does it change the meaning when I use F.Cont. ?


Thank you in advance

[Joe]

She IS working out NOW.
She WILL BE working when you call her (in two minutes etc - future).

[msrdaria]

Thank you!