Timing of error
At conception , the embryo begin duplicating the chromosomes (chms) donated by egg & sperm
to form the 23 chromosome pairs.
If one x chm didn't make it , you will only have a turner pattern (45 chms xo) . If a normal xx pattern starts but an x is lost after several divisions the mosaic is created by having one normal 46xx & another 45xo cell lines replicating throughput the body.