So one thing that you need to understand is that running a link test pushes data to all clients thus producing optimal MU-MIMO results... but in the real world, not every person is going to be using their radio at the same time. You might have specific groups with specific clients that use a lot more data. That group might be reaching 100% frame utilization, but others have lower utilization, so your 450m still could have plenty of capacity overall.
There's a great post HERE where someone basically asks your question and there's some great responses.