AC resistance less than DC resistance in Litz wire
I was testing a wireless charging Litz wire coil at work today and realized something strange to me. The formula I know for calculating AC resistance is "Z = √ (R2 + (XL– XC)2". To my understanding, in this formula, your AC resistance can not be lower than your DC resistance. However, when I was measuring the A6 litz wire coil at 30kHz, I was getting around 58mOhm of AC resistance and 65 mOhm of DC resistance. At 120kHz, the ACR is 78mOhm and DCR is still 65 mOhm, which makes sense then. How is it possible at low frequencies (appears to be less than 42kHz) that the ACR can be lower than DCR? Is there a different formula when calculating this? I am familiar with how Litz wire is woven and skin effect/depth.