There will always be a $5 ($10 for joint memberships) pledge in your savings account. This is what holds your membership in the credit union. 

If your available balance and balance in checking show different amounts, there is most likely a pledge on funds due to a pending debit card transaction. Signature based debit card purchases make funds immediately unavailable by pledging the funds in the account. This gives you a better idea of what money has already been spent and will hopefully help in preventing overdrafts.  

Hold amounts may vary from the purchase amount as merchants sometimes place an authorization for more or less than your actual purchase. Holds on your card from hotels, rental car companies, etc. will also affect your available balance. Balance and available amounts may not match if a deposit made required a hold such as an ATM or shared branching deposit.