9dpo.. BFP but only with FMU???

For some reason I only got a positive (not an indent or an evap, an actual pink line) with my FMU. All other tests today have been negative, even with 3+ hour pee holds and not drinking too much liquid. Is HCG just that much more concentrated in the FMU? Did anyone else only get a BFP with FMU in the early days? I’m only 9 dpo. Thanks 🙏