Why does tallow taste overpoweringly different from saturated fat from the same animal?
Tallow is rendered fat, usually beef.  Off flavors can occur quite easily when rendering fat if it is not done carefully.  Do you know how the fat was rendered? Did you do it yourself?  What was your process?  I'm not sure you are describing tallow in your question, though, because you are describing fat that is discarded by butchers.  So, you'll want to clarify.  I will also add that it is not unusual that fat from different parts of the animal would taste quite different.