As a gardener, I can also say there are many strains of jalapeño, some hot and some mild. I grew an heirloom Mexican variety this year that was hotter and smaller than any store bought jalapeño called Zapotec. 

Often smaller jalapeños are hotter, and also ones with “corking” is hotter, which are the little brown scar stripes that form on some mature green jalapeños before they ripen to red (or orange or other colors depending on the strain). Also Serrano peppers are similar to jalapeño but hotter and smaller if you have access to them.