CVS after positive NIPT for Trisomy 13

So about a week and a half ago I gotta test results from my NIPT and it came back that my baby was high risk for trisomy 13. I went in the following day to do the CVS procedure and got a call last Friday saying the sample was a little small and that they needed me to go in to do a blood draw. Has anyone here ever have a cvs done and had this happen to them?? I just don’t get what blood draw would do when they are testing from the placenta... wouldn’t my blood just give the same result as NIPT?