You should probably begin by looking at the source code for the Serial.begin() function to see what happens when a baud rate is specified.
Specific timing is set up, based on the specified baud rate. Look at what happens when the specified baud rate is not in the list.
You'll notice, if you try to modify the list, that at some point, the resolution of the timer becomes greater than the minimum time required. This limits the speed of serial data transmission.