Mexico trade volume data

I'm struggling with a mystery in Mexico export data. Their national accounts show a healthy growth in real exports (~5.5% YoY growth) while US import data from Mexico deflated by their import price index for Mexico shows a real decline of around 9%. Given that the US is about 80% of Mexico's total exports, this difference is hard to believe. I've so far looked at all possible culprits and haven't found an answer (maquila, deflators, etc).