Table:
|  | Year Ended December 31, |  | % Change |
| --- | --- | --- | --- |
|  | 2019 | 2018 | 2019 |
|  |  | (dollars in thousands) |  |
| Cost of net revenue | $149,495 | $176,223 | (15)% |
| % of net revenue | 47% | 46% |  |
| Gross profit | 167,685 | 208,774 | (20)% |
| % of net revenue | 53% | 54% |  |
Paragraphs:
Cost of Net Revenue and Gross Profit
Cost of net revenue decreased $26.7 million to $149.5 million for the year ended December 31, 2019, as compared to $176.2 million for the year ended December 31, 2018. The decrease was primarily driven by lower sales. The decrease in gross profit percentage for the year ended December 31, 2019, as compared to the year ended December 31, 2018, was due to lower revenue and product mix.
We currently expect that gross profit percentage will fluctuate in the future, from period-to-period, based on changes in product mix, average selling prices, and average manufacturing costs.
Question:
What led to decrease in gross profit percentage for the year ended December 31, 2019?