There could be multiple factors at play here.
You are working with a predictive model. The results are going to be vary based on what you've input into the planner tool; for example, AP model, radio transmit power, environment setup, and overall sizing of the floorplan (making sure its accurate).
On your screenshot, you have client transmit power at 100mW (high; see below), looking only at 2.4GHz, and ≤ 300Mbps. Based on your environment setup, sizing, and AP choice, that is the predicted speed you could see from a 100mW client on 2.4Ghz.
Keep in mind, what you are seeing is predictive and may not correlate to real-world. Typical planning is done on signal strength (for example -65 throughout which will usually yield favorable speeds) and not necessarily speed alone, although you may have this requirement for certain application usage.
Also, you'll want to do your measurements based upon your average client. 100mW is likely high. I typically go with 30mW for planning. I also usually size everything for 5 GHz while planning.