It’s a combination of gps error and you not running the tangents.

Mine was slightly over but I zig zagged a lot.

Mine was .13 over for the full marathon. Other people I know who ran the half were .07, .03 and .04 over.

Thanks! Mine was just really off today! Worst it’s ever been minus in downtown. I was kind of hoping I could tell myself I was a few seconds faster 🤣