Unfortunately, this is common to several colors in the LEGO color pallette. According to this article on Eurobricks.com, The LEGO Group (TLG) made a change in the early 2000’s in how they make their colors: they used to have premixed color granules but abandoned those in favor of a system of white granules with colored dyes added in. This new system allows for many more color combinations than previously possible.
There are certain colors which are more susceptible to color variances than others, especially the above mentioned Sand Green, but also Dark Red, Light Bluish Grey and certain Blues.
The good news is that you can use these color variations to create “weathered” looks, like for example in the Old Fishing Store set where the slight variations in Sand Green color give the building a slightly “beaten” look. This Eurobricks thread gives more examples of how you can create weathering effects in your LEGO models.