I know a woman who had CVS and had a terribly damaged placenta with several different trisomies. She declined a termination because one of the trisomies always results in foetal death anyway and she felt she'd rather let the baby pick its time to go, as it were, but now she has a 3 year old, healthy, son. The placenta had abnormalities which the baby did not. No, i don't know how this works, presumably something to do with RNA versus DNA (Ryn where are you?!) but still a scary (for me) story. They cannot give absolutes in the tests, they can only give a most-accurate guess. Nothing is certain.
This can happen more readily than the reverse ('normal' placenta, baby with chromosomal variations). The cells that form the placenta are 'older' cells than those that form the baby. These cells keep dividing to produce more cells (so the placenta grows) and this is when mutations can occur.

A 'mutation' or change can occur in these cells that does not affect the baby. So some cells in the placenta may have more or less chromosomes than the usual number. There are some changes that the lab people know occur in placenta that would not be present in the baby. Other changes, like an extra chromosome 21 found in some of the placenta cells (but not all) can make it harder for the lab people to say whether the baby may also have some or all cells with an extra 21.

In this situation, a Mum may be offered a follow up amino bc this looks at cells that come from the baby. All these changes are in the DNA. Chromosomes are made up of a long string of DNA wound tightly. When a cell divides into two new ones, one of each chromosome goes to each of the new cells. If the chromosomes don't line up properly, you can get two chromosomes of the same type going in to one new cell, and none of that type going into the other new cell.

If a change occurs in a baby cell, it is usually repaired. If the change is a serious one and not repaired, the baby will not survive.