Those values are average from last 1500 blocks. Every time I look at the chart, confirmation time is shorter for gasPrice 1 gWei, but it should be shorter when gasPrice is 2 gWei. What do you think: is there some issue with this chart itself, or is it a rule of thumb for low gasPrice values?
The speed at which a transaction is mined is dependent on not just the gas price, but also on the overall number of pending transactions and the number of miners currently operating. It's a basic supply/demand model.
If there is not a lot of work pending, miners are not going to sit idle just waiting for the high gas price transactions to come in. They will still work on transactions @ 1 Gwei (assuming it's profitable...the miner can set a minimum price). If there is no other work, those transactions will get mined quickly, reducing the average processing time. Once transactions start coming in that pay a higher gas price, the miners will focus on those to try to increase revenue. At that point, the processing time on 1 Gwei transactions will go up as miners prioritize their work.
To take an extreme example, say (for simplicity) that those 1500 transactions were mined in a 10 minute period. Over the first 5 minutes, 100 1 Gwei transactions were submitted/processed. During this time, everything is humming along nicely without any saturation. Then, in the last 5 minutes 1400 2 Gwei transactions are processed. There's now a backlog causing the average response time of 2 Gwei transactions to be higher than 1 Gwei transactions.