Would you please explain to me why the sentence below is considered incorrect according to http://www.TestMagic.com
"He exercises every day so as to build his stamina."
The correction is as folllows:
"He exercises every day in an effort to build his stamina."
Doesn't "so as (not) to do sth" equal "in order (not) to do sth", i.e. "because"?
Thank you very much in advance.