This just means you need to break up your export into multiple 'read' calls. For performance reasons, the widget API will only return a static amount of values. Meaning that if you requested all of the data points for a given dataport over a week, and there were more than 500 points contained in that slice of time, you will receive not all that data you asked for. This is easily remedied by making another call using either the first or last returned datapoint's timestamp +/- 1. Head or tail, plus or minus, depends on if you asked for ascending or descending order.
Let me know if you have questions about this,