Every website tests different settings. There's a certain standard for brightness, the websites browsed, the software used, and so on. You can't compare battery tests unless all the test parameters are the same.

For instance, Tweakers uses a screen brightness of 180 cd/m² while testing, whereas NotebookCheck uses 150 cd/m². That alone should account for a sizeable difference in test results. I find it hard to tell what 30 cd/m² means in practice.