The first usage you usually see can easily be a little off which is why there may be corrections when calculating direct from the daily meter readings. 
For gas, the calorific value has to be calculated when the details are published which is after a days delivery. 

You have to remember that the daily figure on the app (or online account) is an indication and may not be the true story.

In particular if they don’t yet have the smart meter data for a day they will use an estimate and correct it later when thet do get the SM data, that’s one reason why it can sometimes change.
Sometimes you will see it change between morning and afternoon, depending on when they get the data.

In the end though the daily balance on the app is just an indication, and it’s your actual billing at the end of your monthly billing period that counts.

As an example I had a smart meter sending readings for electricity, but had to send my gas readings manually.
That meant that my daily balances were estimated for the gas use, until I sent a gas meter reading at which point they got changed to reflect what gas I had actually used.
Very occasionally the electricity data may also be a day or two late getting to OVO, again they estimate the daily and then correct it when they have the actual data.
So while the daily balance is just an indication, for the actual billing it all evens out to actual meter readings (if they have them).