I think the efficiency of water condensation. will vary a lot depending. on outside air temperature and humidity. Burning. equal quantities of gas will produce a lot more condensate on a cold humid day then on a warm dry day. Any wind will also reduce the condensation that stays in the catch bucket.
I bet these effects could cause a 100% difference in condensation caught.
Do not know i it still works this way but San Francisco used to charge for municipal steam consumption by measuring condensate from each building.
Bil lD