from what i understand, zephrys assumes you want either kill you opponent (which you couldn’t have done with silence since you had only 1/1s on board) remove as much stat of your opponent’s ‘current’ board as possible (which, again, silence takes away no stats and poly only 2/1 worth) or develop board presence (tyrion)
zephrys doesn’t ‘predict’ super well, yes, killing the two zilliax with holy nova essentially just result in two 3/1 zilliax (assuming you trade your lackeys fir ds) but from its pov, you still removed 9/5 worth of stats from the board in one spell.