alot of reasons , which may or may not be relevant to you.
1. Age - maximum loan duration 25yrs, so due to your age maybe u get a shorter repayment period thus small loan amount
2. Commission or allowance form part of your $5.5k ? It seems HDB loan department usually give a smaller loan if the amount is not just basic salary. I guess allowance and commission can be cut anytime, thus maybe they dont take that into consideration, not sure on this though.
3. You declare other commitment monthly when you apply for HLE. Thus HDB will take away whatever you had declared (car loan, education loan etc) , and based your loan amount on the remaining amount.
Of course there can be other reason, I doubt anyone can give u a definite answer here.
Contact your HDB officer in charge and appeal through him easier.
good luck
-vinz