Why is USDT not equal to USD?

Could you please explain why USDT, the Tether-backed cryptocurrency, is not equal to the US dollar, USD? I've always assumed they were interchangeable, but recently I've noticed some fluctuations in their value relative to each other. Given the purpose of USDT is to maintain a one-to-one peg with the dollar, why doesn't it always match? Are there factors influencing its value beyond just its underlying asset? Or could it be a result of market speculation or other external forces? I'm curious to understand the nuances behind this apparent discrepancy.