You will likely observe changes in your USD spendable balance day-to-day.  Besides spending and depositing activity, your USD equivalent account balance will change dynamically according to the market value of TERN (the underlying cryptocurrency asset for your BlockCard account).  

Your TERN balance will not change unless you make a transaction or deposit, only the momentary USD/TERN conversion price based on market price.  

BlockCard uses United States Dollar Derivative (TERN/USDD) to establish market value. (This is independent of XLM/TERN, ETH/TERN, or BTC/TERN, for example.) Using USDD to establish value helps smooth out some of the fluctuations in the marketplace, but there will still be fluctuations in value based on the value of TERN, trends in the larger cryptocurrency market, and the deposit and spending behavior of users (in particular during times when there is a lot of depositing and spending activity).

Your spendable value on BlockCard is tied to TERN on the BlockCard ecosystem.  We do not use CMC as price value (e.q. XLM/TERN or BTC/TERN).  As users deposit on BlockCard, the value of TERN increases.  As people spend, the value decreases.   This is tied to the USDD/TERN market.  TERN is never issued at less than $0.008, but can be higher based on BlockCard market usage.  TERN can be spent at any price.  We have worked hard to build utility of TERN into the functionality of BlockCard.