Actual Navigation Performance (ANP) is the FMC's estimate of the quality of its position determination. The FMC is 95% certain the the aircraft's actual position lies within a circle of radius ANP centred on the FMC position. Therefore the lower the ANP, the more confident the FMC is of its position estimate.
To be more correct, you should say the lower the ANP the smaller radii of position uncertainty.
The FMC is neither more or less confident, it is just the size of the uncertainty that changes.