This is because of 2 main reasons.Quote from increasenow:
see Dael...how did this guy get so low margin?
1. NYMEX and/or CME substantially rised margin requirements for Light Sweet after Libyan intervention. So right after that the same strangle mentioned in your example cost not $1,000 but nearly $3,000 of margin.
2. When you make a strangle or iron condor (with both sides i.e. calls and puts involved) the total margin would be less or at least equal then the one naked short option. This is because of some kind of insurance (price couldn't get to both call and put at the same time, so opposite side will be in profit anyway). This is SPAN margin in essence.
Look at attached picture. I've made 0.57-0.39 + 0.38-0.22 = $340 of premium with $900 margin required. Just an example.

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