You have hit on an important constitutional distinction here. Constitutionally speaking there is a difference between the local PD and County Sheriff. Sheriff’s are state government while local PDs are not. The constitution only recognized TWO levels: Fed & State. Local PD can not arrest you for registry violations as they do not have the authority to do so.
Some local police departments have decided to take measure on their own accord and pursue their own justice. This is one of the inherent problems with the Whetterling Act as it recognized local PDs as a concerned party. This was of course so that police unions could exercise their fetish toward heroism and not actually protect kids. Overtime from federal dollars are icing on the cake for localities who often assist in compliance checks.