A speedtest is always a snapshot. Various factors - such as the internet traffic, your home network, and your browser - can influence your measurement. That's why it's possible the real speed on your device can be lower than the theoretical speed.
Tip: If you can’t reach a download speed of 30 Mbps in a place where you often use Wi-Fi, you probably need a Wi-Fi booster.
Read more about Wi-Fi boosters here.