The wattage rating of resistors is based on how much power they can dissipate before they damage themselves, and always assumes an ambient temperature of 25C, but the specs rarely tell you what the actual temperature is. Small, high wattage resistors are made of material that lets them get VERY hot.
A bunch of resistors, all dissipating power in a shared space will warm each other, and their surroundings, up, and reduce the overall rating, but in practical terms, if the ones used are not going an unhealthy colour, they are OK. And their surroundings. I have seen higher powered resistors cook the plastic insulation on nearby wiring.