This is because some historic changes in observing practices, site moves and changes in exposure are undocumented. (bom.gov.au)
Moms are instructed to observe this practice from their child's first day home from the hospital and then to extend tummy time as their babies grow older. (romper.com)
This small change was made to mitigate the effects of suspect data due to poor observing practices at the station. (stormbruiser.com)