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 -> Basket Settings
Scroll down until you see the setting Round Prices During Calculation.
Make sure to have this setting disabled.
Your shopping cart values should now work as expected.