The Guardian claims that an official claimed that they were told by AZ that the discrepancy is due to European countries having paid for R&D, but anybody in the chain could have been lying.
[0] https://www.theguardian.com/world/2021/jan/22/south-africa-p...
[1] https://www.reuters.com/article/uk-health-coronavirus-safric...
At least you are getting it. We Europeans seem to have pre-funded production and now it turns out much of it is shipped to higher-paying customers first, and we must wait until "production issues" are solved.
Also, put another way, those EU countries are probably getting charged less than the average costs to actually make the doses themselves. That's fine.
An issue is that R&D is a fixed cost rather than a variable cost, so determining how the R&D costs are to be split among AZ's customers is pretty hard and potentially prone to abuse. Are they to be split among all expected customers before the pandemic ends? All expected customers during the lifetime of the vaccine? During the lifetime of this family of vaccines?
Given that apparently it makes up the majority of the price, it's quite an important detail.