Anyone ever had challenges in the accuracy of data usage as presented by the provider (Spark in my case) and what you believed to be used?

I used to have a 9GB plan, occasionally came close to the limit but usually had some buffer at the end of each month. Recently moved to the unlimited (as it was cheaper than my previous plan), and in the space of 10 days I appeared to have used 18GB. You can't use it as a hotspot (or couldn't before they started offering add-ons, but i digress...) and so it is purely web and app usage on my phone; iPhone 7 and have WiFi Assist turned off.

I'm usually on a wireless network, whether at home or at work, or even the free spark wifi. I would say the 15 mins into work, the 15 mins home from work and the odd time I am walking through Welly CBD is when I am purely using mobile data. As mentioned, WiFi assist is turned off so when on WiFi, mobile data is not kicking in.

I reset my data usage on my phone, have been solely on WiFi, only avoiding airplane mode so i can receive texts and calls. My phone tells me that it has used just over 1MB of mobile data in the last 24 hours. My Spark app says I've used over 1GB.

I have asked Spark to monitor usage, but they haven't come back to be yet, but I was wondering if anyone had experience of something similar? I am also wondering whether the recent IOS11 upgrade has got something else going on, so i am not assuming it is purely at Spark's end, but this detective work is going around in circles. 

Any help appreciated.