It's a recommendation, the idea is to ease design when design software is not available and people making the decision do not have good understanding how to actually calculate/simulate these things.
Actual calculation is done based on temperature-compensated worst case open-circuit voltage of the panels. That must remain below the maximum input voltage of the inverter. In the past it was common to actually design PV installations using design/simulation software, where you choose panel type, inverter model, location of install, and it would immediately say PASS/FAIL, today it's more usual just to sell people something which probably works or not, so mistakes happen, like here a company sold hundreds of installs with so few panels that they don't ever reach minimum MPP voltage and sometimes not even startup voltage, it would have been a 15-minute napkin calculation for any professional to see it is not going to work.