Gross profit margin

Can you please advise the meaning refer to the different figures : -1% and -1.50 % as follows ?

Thank you.

1. Gross profit margin

2015 = 68 %

2016 = 67%

Drop – 1 %

2.Net profit margin

2015 = 5.00%

2016 = 3.50 %

Drop – 1.50 %