Some have already responded in the thread.

When you increase difficulty on a game, numbers mean monsters have more health and/or hit harder - numbers on stats and dmg increase. Mechanics are much harder to implement but greater satisfaction, instead of making monsters harder you make them smarter, you add additional mechanics like special attacks, better positioning, extra monsters can be considered both numbers and mechanics depending on how it is done.

Do you need to hit the early monsters 5 times in the head to kill them or monsters now are not as easy to kill because they try to flank you and explode or something.