New air m3 battery

Hi, I am new to MacBook or Mac in generell and bought a new MacBook Air M3 512/8gb from a I would say serious store. Sometimes the battery (I am studying all day atm) lasts from 100% to 20% for about 8-9 hours display time, middle to low brightness, using Safari, Goodnotes and sometimes ChatGPT. But sometimes the battery was from 100% to 30% in just 4 hours with same settings. Honestly, is that normal? Is the 18 hours Apple gives on their website only for videos and not day to day use like safari etc? The Mac is new from a store, settings say it was charged 1 time some months ago, and the laptop was active 2 weeks before I bought it.