As distinct from yoga, tai chi is not so much concerned with increasing the range of motion as it is with cultivating freedom of movement within whatever range of motion exists. This is the difference between fluidity and flexibility.
This is just an ordering of priorities, and does not reflect a rigid hierarchy. In either practice, both should be emphasized, the difference is in which one is emphasized first.
Why emphasize fluidity before flexibility? Because effectiveness generally depends more on responsiveness than capacity.