Most probably it's because your SMPS uses the flyback topology.
With this topology, energy is charged into the magnetic core from the primary and then discharged into the secondary. Voltage regulation is done by adjusting the amount of transferred energy. While the charging phase, the full rectified input voltage is applied to the primary, and the secondary rectifier blocks that voltage. Your additional rectifier works the other way round, so your circuit receives the primary voltage multiplied by the transformer ratio. In a flyback converter, this voltage is not regulated nor has anything in common with the regulated output, it just depends on the input voltage and the transformer ratio (plus ringing).