When using Quantity Based Pricing the system interrogates the pricing bands, and the quantities and divides them down.
In some cases this can lead to rounding taking place which causes incorrect formatting within the shopping basket.
This can be fixed per Storefront.
Login to your Storefront as an Administrator.
Go to Configuration -> Settings -> All Settings
Use the filter in the SETTING NAME column to locate a setting named: shoppingcartsettings.roundpricesduringcalculation
Set the value of this setting to FALSE.
Your shopping cart values should now work as expected.