A lot of people know enough not to build farms and quarries but often will make half lumber and half iron for resource fields. On the surface this looks like a good idea since you need both of those constantly for troop making and both are similarily priced in the marketplace.
But look closer and you see why lumber mills are better. A level 9 lumber mill requires 450 in population. A level 9 iron mill requires 1,125 in population. Lumber is the more efficient resource for your city while still leaving some population available for troop making. Yes, you can zero your production, make your troops and then put production back up but production is lowered until its back up again and its a pain to do that each time. For those that run heavy troop making cities with 17 or 18 barracks this is most important.