Check whether at the center level, under the Invoice and Receipt section, you have selected the Balance with tax included setting.
When this setting is selected, the balance shown on the communications will include the tax amount and so will be different from the balance shown on the system.
Here is an example showing how the numbers are calculated.
Membership price: 100
Tax on membership: 20%
Tax amount: 20
Credit amount (Total credit amount accrued through Membership collection): 200
Credit amount (Total credit amount accrued through Membership collection+ Tax): 200 + 20 = 220
On Invoice 1
Credits used: 48
Credit balance (shown on page): 200 – 48 = 152
Credit balance (shown in communications): (220 x 152) / 200 = 167.2