There may be many reasons for that... for example:
- The line protection is specified at 275 V (nominal operation voltage), however, during transient surges or other events this voltage is exceeded, provably clamping at 300 - 350 V, this is why most of the elements are rated at 400 V. This circuit may be designed for real life applications where you can expect line overvoltages, surges, etc, not a perfect 220 V RMS Sinewave input! In addition, if you want to meet some EMC requirements you have to test the device (and survive) with voltage surges, burst, etc.
- Capacitor aging depends on working voltage, using higher voltage capacitor would reduce derating compared to a lower voltage one...
- There may be other reasons.