Our total support figure is an itemized list of what it costs us to live and work in Japan as missionaries. Some of these non-negotiables include US and Japan taxes and health insurance, as well as visa and passport fees. We also have to pay monthly rent and yearly school costs for our family. These have to be paid in full whether we are 100% supported or not and when we aren't fully supported we start to have deficits in these areas.
Another reason that is hard not to be fully supported is that we have to cut costs in other areas of life and ministry which ultimately makes it harder to effectivly work and live in Japan. Some of these cuts might be funds for conferences, outreach, car or transportation, computer or monthly "living allowance" (a frugal set amount that each misisonary gets per adult and per child for "personal" costs like clothes, food, etc). 
But in the end, it all comes down to trusting, trusting God to supply for our family. Whether we trust God here for our needed pledged monthly support before we leave or whether we trust God in Japan for our needed support income, we are still trusting...trusting in our faithful God to provide for all we need to be His witnesses in Japan.
Another reason that is hard not to be fully supported is that we have to cut costs in other areas of life and ministry which ultimately makes it harder to effectivly work and live in Japan. Some of these cuts might be funds for conferences, outreach, car or transportation, computer or monthly "living allowance" (a frugal set amount that each misisonary gets per adult and per child for "personal" costs like clothes, food, etc). But in the end, it all comes down to trusting, trusting God to supply for our family. Whether we trust God here for our needed pledged monthly support before we leave or whether we trust God in Japan for our needed support income, we are still trusting...trusting in our faithful God to provide for all we need to be His witnesses in Japan. 