1. That was what her creator meant for her to be and he was quite clear when he stated as much.
2. During the Golden Age, she towed the sun back into place to sort of back up this 'really really strong' concept
3. At various times throughout the history of DC, she has been the one to fight him to a standstill. Batman pointed out that she was the only one able to stop Superman if he were to go rogue.
I'm of the 'she's definitely an alpha hero' school of thought myself but think she should be a hair less stronger than Superman but more skilled.