Burnthesheep does a good job explaining the reason.

There is a core concept in training to power called Functional Threshold Power: this is notionally the average power you could sustain in an all-out hourlong effort. People often test their FTP by doing an all-out 20-minute effort at 5% above their expected FTP. You're finding that a 20-minute effort at 8% above your usual ride is really hard, which seems to tally. I think you'll find that if you stick with it and train regularly, you'll make rapid gains before you start to plateau.

You can do an endless amount of study on structured training and training to power.