Temperature problems

[Geckolisa]

I got my new gecko yesterday,and im having trouble with temperatures.I have a microclaimate mini stat 100,and a digital thermometer.Both probes are flat on the ground and next to each other.Inside my geckos warm hide.I have the stat set to 90f,and it goes up ok,and switches off when it hits temperature,but the thermometer reads a temperature of about 86f when it goes off,and then it keeps dropping to the high 70's,before the mat stat comes back on again,and the same thing happens.What am i doing wrong? is it the themometer?I admit it does say aqarium digital thermometer on it,so would that be the reason why its so inacurate?