20 seconds? I've seen blocks a few seconds apart in the past.
Every block gets solved in 20 seconds. Think I am wrong? You don't understand math.
For each block, go to a moment in time 20 seconds prior to its solution. At that moment, the network has accomplished zero progress on the impending solution. However, in the coming 20 seconds - it finds a solution.
A solution doesn't get easier as time passes doesn't depend upon. The network doesn't advance and close in on a solution as the clock ticks.
The same error in statistics is made for those people who think that 10 rolls of a die without getting a '6' increases the chance that a '6' will come next. It doesn't. The chance of rolling a '6' is the same whether or not 10 non-'6's occurred previously.
The state of the miners 20 seconds prior to a solution is identical to the state of miners which have been working for one hour. Thus, the probability that a solution will come in 20 seconds is identical for the case where a solution was just found, and for when the miners have been working a long time. Miners having been working a long time do not increase the probability that a solution will come in the next 20 seconds. Thus, every block was solved in 20 seconds - it is just 20 seconds prior to the moment the solution came.
Idiots - find it incredible that a block was solved in 20 seconds - they all are.