It's very common. Wired ethernet can actually deliver the advertised speeds. So if you have a 100M connection, then it can actually deliver 100Mbps.
Quoted wireless speeds are largely wishful thinking, and are based on the best possible set-up, which you will never have in the real World. The speed you actually get depends on how good your signal strength is, and how many of your neighbours' routers are broadbasting on the same channel.
On the 2.4GHz waveband, the channels are crushed up together so much that a transmission on any one channel will interfere with the adjacent ones. That means that there are only three completely separate channels, and if you're in an urban area, it's very likely that a neighbour's signal is overlapping yours.
The 5GHz band is a lot better for congestion, but doesn't have the same range.