weird data allowance

When i check my iD mobile app, with my data turned off, it display my data, for now lets kust say it is 975mb. When i check it with my data, it shows 925mb in this case and it always shows exactly 50mb less.
I use a Huawei p20 lite running Android 8.0.0 and i use the iD mobile app to check my data.

Im not sure whether it is a glitch or something but I would appreciate it if someone can find a solution to this.
Thanks in advance,
Mr D

P. S when i use the 7777 costumer care call, it also say the same thing, when data is on it says 925, when it is off it says 975