Vodacom acting like MTN

I find it strange how my data keeps disappearing, bought 1gig on 22 oct and it’s almost depleted a week later when everyday for 8hrs I’m on WiFi from work, this very month I had bought 2gig which made it only to the 22nd oct, help me understand how 3gig is depleted with WiFi usage during the day, data off when I sleep from 10 to 6am and minimum internet usage in the evenings due to having a child, kindly explain.