Is someone able to explain why the block reward changes (Not the halving I know about that)

In 2017 and 2018 the block reward varied between 12.5 and 15 bitcoins rather than the 12.5 as assumed and again in 2021, is this some sort of compensation for mining difficulty variability ? If someone knows why would be greatly appreciated. Picture below explains (https://mempool.space/graphs/mining/block-rewards)

My understanding that the block reward was fixed per halving schedule, this is obviously not the case?

mining block rewards since 2009

last 6 months

Any input appreciated, cheers.