You have probably compared Broad match on Keyword Tool External with results in Keyword Planner tool - this would account for a huge differences you are seeing. Keyword Planner tool shows numbers for the "Exact match" type (bold emphasis mine):
Average monthly searches ("Avg. monthly searches"): The average number of times people have searched for the exact keyword...
If you compare Keyword Tool External using Match Type = [Exact] with the results in Keyword Planner tool, then the numbers are fairly similar.