Why does my margin balance show up lower than the value of the assets in my account?

Although you can cover your margin requirement with a variety of tokens other than rUSD, their value is taken into account with a haircut, i.e., their value is reduced by a percentage that depends on the particular token.

Haircuts cover for the risk that your tokens reduce in rUSD value by the time they are needed to cover losses. This would leave your account insolvent. You can think of the haircut as a sort of margin requirement in reverse: it’s the amount of rUSD Reya can safely credit your margin balance after taking price fluctuations and volatility into account.

If you want to know the actual value of all your collateral without haircuts, the number you are looking for is the account value or equity, which you can find in your portfolio dashboard.