The Provisional data got replaced with the final official data. If you read GW's posts from month to month, the corrections can be quite sizeable.

I did ask him once why the corrections are so large when they have all the data already but I can't remember exactly what he replied (I recall it was along the lines of "good question" )

Maybe worth asking the MetO direct. The reporting stations that contribute to the figures must be accountable publicly for their use of revenue and either they all got it wrong to start with, or one got it very wrong, or neither, in which case the adjustments should be explicable as they are also made in an official capacity in public-money-funded time.

The MetO in my experience are very good at responding to public questions and seem to have dedicated staff ready to do this.

Also in my experience, the actual question doesn't necessarily get answered.