Daily activities can make a huge impact. I am running around a lot at my job, which is probably another reason why I need a huge surplus, just like quadancer.
You've got to just adjust your own intake periodically and see what happens. Also, as you gain weight, you'll need to reevaluate your calorie needs as well, since your maintenance requirement will go up. It's not an exact science and it takes a bit of time to figure out... However, eventually, you should be able to figure out roughly how much of a surplus you need to bulk and it should stay more or less the same, as long as you don't do something drastic like change jobs, drop half your routine, get injured, etc.
You said you are doing 600 over on workout days, what about non-workout days? Remember, you are building muscle those days too, so you'll want a surplus on those days as well.