I just made a top up, but why is my withdrawable balance smaller than my top up amount?

In accordance with the previous point, the way to calculate the Withdrawable Cash is by subtracting the smaller amount between the Balance or Equity with the Initial Margin that has been used.

If before topping up your Margin Level is below 100%, then a portion of your top up amount will be used to make your Margin Level an even 100%. If your Margin Level has reached 100%, then the remaining funds can still be withdrawn at any time.

This can happen if you have unrealized losses in your leveraged positions (2x or Day-Trade).