MacOS dictation inserting random words

I've changed over to dictation and voice control on my Mac. It works fairly well, but tends to insert random words. Sentences often begin with 'I' or 'And', and sometimes whole phrases get pasted in, something that I typed four chapters ago. I'm doing this into scrivener, which simply uses Mac dictation. Is this normal, is there any way to adjust it. It's not fatal, but certainly annoying.