Are you, by chance, a Creationist? This is exactly the sort of deliberate misuse of terms in which they indulge when trying to obscure the distinctions between their mythological stories and scientific theory.
The two words mean quite different things. To use one when you mean the other is obfuscatory. Pray attend, grasshopper:
vs.
The one is made in the absense of evidence, the other is derived from evidence. A conclusion can be incorrect but it is not the same thing as an assumption.
Now, I will grant you this much. It is common in vernacular useage to dis the conclusions of one's opponent by calling them "assumptions." I do this myself from time to time. It is, however, just a form of insult.