Hi Ms. Muddle,
That's a great question and I think the answer is...both are or can be true.
People with ADHD often struggle with memory whether it originally "registers" or not. In the former, we often see people who have difficulty with recall or sustaining it in our working memory. We might enjoy a wonderful movie but forget the plot and main character the minute we leave the theater. Or we can enter a grocery store with the intention of buying three things, but leaving with only two.
We can also have problems with memory, as you've pointed out, because things might not have registered to begin with. Why? Because we are often so distracted that the information never gets to our brain in the first place.
So to answer your question, yes...I'd have to say that both can be true.
Terry