Confused on buying power for short positions

Ideally, I would like my system to operate as follows:


1) System will go long and short the QQQQ

2) Starting amount of $10,000.

3) Use maximum buying power without leverage 90% of the time

4) Occasionally, use max. leverage

The problem is that after initially opening a short position (in the QQQQ) of $10,000, the buying power in the model account status (even after updating the #s) does not show $10,000 as I would expect given 100% leverage.

Also, if I go ahead and open another short position using the ‘buying power’ that is displayed in the model account status (say $5000), then after the position is opened, the buying power does NOT go to zero as would be expected and thus, apparently, I could open ANOTHER short position using the new ‘buying power’ (say, $2500).

This is confusing and I am hoping someone can clarify the proper way to use buying power when opening long short positions in an ETF such as the QQQQ.

Thank you