' Why is the UTC time incorrect while time of week (ITOW) and week are valid?'

Related to the earth rotation slowing down, UTC time is corrected with so called leap seconds. These leap seconds are not included in the initial UTC time and become only available after about 12 to 15 minutes of GPS reception. Once the leap seconds are known, the UTC time is corrected accordingly.

In 2010, UTC time has to be corrected by 15 leap seconds, which means that it is important to wait unttill the UTC flag is valid before synchronising the MTi-G data with other UTC-referenced data.