Couldn't go or couldn't have gone - real/hypothetical ability

Hi. I'm doing a Macmillan's Destination now (C1 & C2 one) and got puzzled while doing some exercise. It was about modal verbs and theory said that when we talk about lack of past real ability, we use couldn't; and when we speak about lack of past hypothetical ability, we use couldn't have. So there's an exercise and one of the sentences read as follows (answer is in bold):

Dawn couldn't go to the barbeque as she had to work.

And several lines later, inside another exercise, we have the following:

We couldn't have gone to the concert anyway as Jamie wasn't very well.

I'm not sure I get these very well. Could someone tell me what's the difference and why in the first case it's lack of real ability and in the second one it's lack of hypothetical ability? Or is it something else?