This site is being deprecated.
Please see the official X‑Plane Support page for help.
That restriction may just be the maximum airspeed that the aircraft would allow into its auto-throttle setting. The value in the panel is in Indicated Airspeed (IAS). At FL300 (or higher), for IAS of 393, I can imagine the equivalent True Airspeed (TAS) would way exceed the limit for the B737-800.
For example, if you want to cruise at 450 kt, at FL300, the IAS may be around 285 kt—depending on atmospheric conditions.
The explanation can be found here: https://www.boldmethod.com/blog/lists/2019/11/the-four-types-of-airspeed-and-how-each-works/
The speed that your airline app was showing is in True Airspeed, which is how fast your plane is really--truly--slicing through the air. In short, what is indicated in the 737’s Primary Flight Display is Indicated Airspeed (as the name implies); which is also what you would enter into the autopilot.
What framerate are you running at? Your sim could be suffering from this “Slow motion” syndrome…
This post is getting too lengthy and off topic. I’ll MP you to follow up…
This site is no longer being actively maintained.