If blocks are solved every 10 minutes on average, then shouldn't the average answer to the question "How long do I need to wait until the next block is solved?" be 5 minutes?
If I roll a six-sided die until I get a 1, then the average number of rolls between 1's is six rolls.
Does that mean that if I walk into a room where someone is rolling a die, the average answer to the question "How long do I need to wait until the next 1 is rolled?" is three?
No.
The die has no memory of what has been rolled recently, or how many times it has been rolled since the last 1. The average will always be six rolls starting "now", no matter when "now" is.
It's the same with mining.
Shortly after a difficulty adjustment, he average is 10 minutes from "now" no matter when "now" is.
That average shrinks or grows depending on how much hash power has been added to or removed from the network since the difficulty adjustment, but the average time between blocks and the average time until the next block will always be the same.