The glass transition is typically defined as a range because of the way it is typically measured - there is a region where the behavior is neither that of the solid or of the liquid. The reason for that is because the measurement is typically done on a faster timescale than the melt can react at those temperatures, and so the structural changes taking place lag until it reaches the point where it catches up - where we then can clearly say it’s a liquid.

What throws a lot of people is that glass does not have a clear transition temperature; instead of water freezing to ice, which happens at 0 degrees Celsius at standard pressures and without additives, the temperature at which the melt transitions to a glass depends on the rate at which you cool it. It will continue to rearrange its structure to find the lowest energy arrangement - just like a liquid - until the re-arrangements take too long and the atoms no longer have the kinetic energy available to move (too large of a region would have to shift at once). At that point, it’s a solid. (Fun fact: this point can also depend on the measurement technique and time scale.) So the glass transition is a kinetic transition instead of a thermodynamic transition like freezing ice.