Hi rakeshd347,
GMAT SCs tend to follow certain style "patterns"; one of those patterns is that, in general, if two answers APPEAR to be correct, then the shorter answer tends to be the correct one. Now, THAT rule is still based on the idea that the GRAMMAR is correct. If the grammar is incorrect, then it doesn't matter how long or short an answer is.
In the case of "considered" vs. "considered to be", either one is acceptable. The good news is that if this issue occurred on a GMAT SC, then there would be other grammar rules that you could use to determine which answer is correct. Ultimately, you won't be left with ONLY this rule to help you find the correct answer, so you shouldn't worry about it.
GMAT assassins aren't born, they're made,
Rich