Sometimes its because the test wasn't done properly , or there are other things affecting the specimen for example if it is damaged or contaminated....or there is not enough 'blood' to give a reading or it is only in one of the days....lots of possibilities but usually means they will either ask for a new set of poo or will go to the next stage of testing
Don't worry, this happens. If your mum is slightly constipated or has piles and there is a little blood in her stools, this could give a reading that the blood is possibly a sign of bowel cancer. They will probably give your mum another couple of tests until they are sure she is OK.
Happened to me and I did 2 more separate sets of tests and they came back clear, That was 2 years ago and had another test sent this year which was clear, Better to be safe than sorry for your Mum.