How accurate and reliable is MySpark's broadband usage history?

We're on capped rural broadband so check and manage our usage closely, but this morning, MySpark is showing 19Gb usage at 12am this morning.

Part of this may be my wife's usage of HD Netflix late yesterday (say 2.5 hrs @ 4GB / hr) but the other 9Gb is inexplicable on the face of it particularly as upload backups to iCloud are switched off and i/m not aware of any bulk programme or app updates overnight.

Spark aren't able to analyse the data usage spike for reasons of data privacy, which I understand and up to 1/07/20 that data cap has been lifted by Spark anyway so we're not at risk of exceeding the contract cap.

Networks are securely passworded and we're very rural so I'm pretty sure our bandwidth isn't being "used" by others.

But all seems a bit strange.

Has anyone else experienced this?