Flatten surface gives an unexpected result

Im still messing around trying to get parts sorted for laser cutting on this tank.  When I flatten a fillet I get an odd shaped piece and Im not sure what Im doing wrong.

To get where I am I have selected the fillet and chose surface flatten and then choosing a point to flatten from.  Some parts it works as I would expect it to but these cirved fillets is giving me something I do not understand.

Thanks, Daniel.