I created a transaction with gas price 1.1 GWEI. It has been pending for 3 hours, and yet - during just the last 75 blocks - several 1 GWEI transactions have been included.
In other words, gas price is clearly not the only determinant of which transactions are picked to be included in a block.
It seems to me that (at least some of) the Ethereum miners might prioritize older transactions over transactions with higher gas prices.
Box Plots of Gas Price in GWEI for the last 75 blocks:
Notice that in the Box Plots, there seems to be two types of blocks. Those that include 1 GWEI transactions, and those that don't.
My hypothesis is that there are two main types of miners:
Those that mine the transactions that pay the highest gas prices
Those that mine the transactions (minimum 1 GWEI) that have been waiting the longest.
Considering how common "1 GWEI-blocks" seem to be - who are these miners? And why are they picking old pending transactions, rather than those that pay the most?
Disclaimer: I don't actually know that it is the oldest transactions that are included in the "1 GWEI-blocks", I'm just assuming this, since they are clearly not picking the highest gas price transactions. Another explanation would be that these miners pick transactions randomly.