Ranges suddenly increase

I have a TAG and an ANCHOR and I am trying to get the range between them. I am using Single Side Two Way Ranging, the TAG and ANCHOR are 1m apart. I have an antenna delay set to 16436 on both the tag and anchor. At first i get good values ranging between 0.8 - 1.40. However, after around 5 - 10 minutes the value go to 2m and stay there. I do not do anything I am just finding the range.

Does anyone have any idea as to why this happens?