Is it normal that i'm not getting the advertised internet speed?

They all lie about their speed. If you call them and bitch about it they'll sometimes try to fix it. One time my internet company told me to take a speed test from their own website which showed faster than independent sites 😂

Stated internet speeds have always been something of a lie. I'm sure that if you read your ISP's contract carefully, you'll find that NBN promises to provide a maximum download speed of 25Mbps, but there'll be some qualification in there saying that the actual speed you get will be affected by the volume of traffic on the network.

It's no different to how the speed limit on a road may be 110 km/h, but the actual speed you can travel at will be affected by the number of other cars on the road and lanes being closed for maintenance. If there was literally nobody else on the NBN lines you use and everything in the system was working correctly, then you'd probably get your 25Mbps. But because you're sharing those lines, you'll only rarely hit that speed.

I know nothing about NBN other than it's in Australia, but I know that many ISPs all over the world have had huge problems with network capacity over the last year, and particular problems with their domestic lines. Their networks were set up based on projections of usage patterns for commercial and domestic users that didn't work well when a lot of people started spending much more time at home during the day and using the internet for work, education and leisure.

But, really, unless you're constantly downloading huge lumps of data, and multiple people in your household are watching videos and video-calling simultaneously all the time, you probably wouldn't even notice the difference if your real-world internet connection was bumped up from 22Mbps to 25Mbps. That's only an 8% improvement. Only real geeks obsess about that level of benchmarked speed increase in their tech; most people wouldn't be able to tell the difference.