USDT/USD Price does not seem to reflect volume well.

USDT:USD Price does not reflect volume well. Why is the price so solid?
Seems increase in value one would expect with such an increase in demand is being transferred elsewhere.
Does this mean that Tether is not a currency of any type but a fixed value token ... perhaps with no inflation?
How does this work? Which of the other crypto coins can be expected to behave the same way?