I have very few days of stock left but the system says I have excess stock or a reorder due date further in the future, why is that?

It's because you've already placed orders and you have an on-order amount that the system is taking into consideration. 

Stock left only includes the stock on hand, but the excess stock and reorder due date takes into account items you have already on order. 

In terms of order planning, StockTrim also includes the on-order quantity. To plan future orders, it makes sense for current on-order items to be taken into account if they’re already on their way, so if they will cover the demand you don’t need to order more.