@OP, it's normal for [non-regulated] wallwarts to be outputting 4-5V
above their rated voltages for "small" load currents. The voltage only
drops to near the rated voltaqe for load currents near the max for the
given device.
So-called regulated wallwarts should always produce near the rated
output voltage, however.
A 6V non-regulated wallwart will probably work fine as long as the total
Arduino load currents are small.