The number of bitcoin in a block is also the percentage of all 21M bitcoins to be mined over a 4 year period.

Example: 50% of all bitcoin was mined in the first 4 years; block reward was 50 bitcoins at that time. 25% got mined over the 4 years reward was 25 bitcoins, etc.

Next halving puts reward at 3.125 bitcoin per block. This means inflation rate of 0.78125%…this is less than gold.