[Proj] some problems with transformation.

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Hello, I use cs2cs utility and try to transform a point (484236, 5006526) using the following command line:
+proj=utm +zone=15 +ellps=GRS80 +units=m +to +proj=aeqd +lat_0=-80 +lon_0=-100 +x_0=0 +y_0=0 +ellps=GRS80 +units=m 

The result is (631024, 6129620).

Then I add flag -I to my command line and try to transform point (631024, 6129620). The result is (329673, -2729005).

Please, could you explain this. Thanks. 

P.S. I suppose that inverse transformation is rigth (631024, 6129620)->(329673, -2729005).
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