Long vs Short Margin BuyingPower

Scenario:

  • Paper trading - standard setting (Levarage x4, DayTradingPattern = true etc)
  • 400.000 USD BuyingPower
  • Ticker = QQQ
  • Testing in both C# API and web interface.

Issue:
When entering a long position maximum quantity of stocks is 1060
When entering a short position (same price) maximum quantity of stocks is 1030 (not 1060 as in long position)

Question:
The maximum BuyingPower is about -3% when when shorting on margin. What is the logic here?

Help is appreciated, thanks!