The best authorities give the Prussians 87,000 men, and the French 78,000; but the latter estimate includes the corps of Lobau, 10,000 strong, which did not reach Fleurus till dark.
I follow Houssaye's solution of this puzzle as the least unsatisfacty, but it does not show why Napoleon should have been so perplexed. D'Erlon debouched from the wood of Villers Perwin exactly where he might have been expected. Was Napoleon puzzled because the corps was heading south-east instead of east?
Delbrück ("Gneisenau," vol. ii., p. 190) shows how the storm favoured the attack.